vayomar a-donai im-na matzati chen be'eyneycha - My master, if I have found favor in your eyes
Avraham included all three men in his address and invitation saying to the senior one amongst them "If I have found favor in your eyes (singular)." The assumption then is that the word a-donai is secular and does not refer to someone representing the Divine. (Rashi)
The problem with this kind of approach is the vowel kametz in the word a-donai. Wherever we find this word vocalized in this manner it always means "my Master," i.e. someone is addressing G-d and that is the reason the plural is used. The reason for the plural is that celestial beings always appear in the plural such as e-lohim, malachim, etc.
It is possible to argue that Avraham addressed only the archangel Micha'el when he said "a-donai." Micha'el, in his capacity as the angel representing the Attribute of love and kindness (Mercy), was the direct superior of Avraham whose outstanding characteristic was this very virtue or Attribute Chesed. It was no more than right that he should address him as "my master." This would also account for the fact that Avraham first offered water and subsequently butter and milk without mentioning wine as part of the meal at all. (Wine is considered as part of the domain and emanation Gevurah, the opposite of the emanation Chesed.)
According to the discipline of vocalizations there are seven gradations in the vowels (not including semi-vowels which are not audible but which nonetheless are not part of the consonants). The vowel kametz ranks as highest of these seven levels. It is followed in descending order by patach, tzere, segol, cholem, shurek, and chirek. The entire Torah is structured around these seven vowels which affect pronunciation of the words. They are also known as s"even syllables, or seven sounds, concerning which David said in Tehillim 29, known as the hymn in honor of the giving of the Torah, "the voice", or sound of G-d occurs seven times. This is also the meaning of Shemot Rabbah 28:4 that the Torah was given with seven kolot (sounds). Concerning these seven sounds, Shlomo said in Mishlei 9:1 "she has hewn her seven pillars." These seven sounds are the foundation upon which the whole structure rests.
The difference between the vowel kametz and the vowel patach (otherwise found in the word a-donai) is merely a single dot, and usually such a dot is perceived as an allusion to the original "dot" of matter which was the beginning of the creative process of this universe (in Kabbalah, the dot is equated with the letter yud, itself an allusion to the Ten Emanations). This is the mystical reason why such a dot (in the way we write the vowels) serves as seven different purposes. When you place such a dot on top of a consonant it produces the vowel cholem. When the dot is placed in the middle of the consonant vav the result is the vowel shurek. If the dot is added to the vowel patach, the result will be the vowel kametz. If the dot is added to the vowel tzere the result is the vowel segol. If the dot is added to the semi-vowel sheva the result is the vowel kibbutz. So you have seven different vowels merely by changing a single dot.
Concerning the letters themselves. If a dot is inserted inside the letter heh it turns it into a chet. If a dot is added to the left side top of the letter vav it becomes a zayin. If a dot is added to make the base of the letter chaf protrude, it turns into the letter bet. If you add a dot on the right top of the letter resh it turns into the letter dalet.
We can understand therefore what the Sages mean when they say that an extra dot or a missing dot is liable to destroy the universe. (Sotah 20)
Although, at first glance it appears that there is oly a minute difference between spelling the word a-donai or a-donoi, (in the Sefardic pronunciation this difference is not even audible), there are profound differences in the meaning of the word as a meaning. Here are a few examples of where such minor misspellings have a profound effect.
Yehoshua 3:6 speaks of the Aron haBrit (the Ark of the Covenant). When spelled (correctly) with the vowel patach, the word aron (ark), is a possessive of the word habrit, G-d's Covenant. If spelled incorrectly with the vowel kametz, this would convert the ark into being the covenant.
In Shemot 23:20 we have the pasuk "Hineh anochi shole'ach malach" - behold, I send an angel. The word malach is vocalized with the vowel kametz as it is not in the possessive clause. Whenever the word malach is in the possessive clause it must be vocalized with the vowel patach. At the end of a verse or at the cantillation etnachta, the vowel patach is always changed to kametz to indicate that the word is in its own right and is not a possessive clause which would be presumed otherwise. The patach always points to the word which follows it, making the word with that vowel at the end secondary to what follows.
(Mateh Moshe, on laws of prayers who quotes Rabbi Bachya, adds that when the word a-donai is spelled with the vowel chirek at the end, it means "my 'personal' master/lord," as opposed to acknowledging that "He is the ruler of the whole universe.")
The reason that in our verse you do not find the word a-donai vocalized with a patach is best understood by remembering that if Avraham had indeed addressed only Micha'el he would have had to say adoni, "my (personal) master/lord." Neither the vowel patach nor the vowel kametz would have been appropriate. It would be incongruous to vocalize the word a-donai when used as a reference to G-d with the vowel patach, as this would imply that G-d is in some kind of subordinate relationship to anyone as suggested by the possessive clause represented by that vowel.
In short, if someone exchanges the kametz under this word for a patach he cannot escape being guilty of one or two things, he is a heretic, or he completely distorts the meaning of the verse in which this word appears.
Bereishit 18:23 can be used as an example. If someone were to place the vowel kametz instead of the vowel patach under the letter heh in ha'af tispeh (which is perceived as turning what follows into a question), he would completely distort the meaning of the whole verse, turning it into a statement, i.e. an accusation against G-d.
Another example. In Iyov 8:3, in "ha'E-l ye'avet mishpat" - does G-d pervert judgment?, the letter heh is vocalized with the vowel patach. As it stands the verse is a question and means, "is it possible that G-d will pervert judgment?" If you were to place the vowel kametz under the letter heh in the word haE-l in that verse it would turn the entire phrase into a statement denying G-d's justice, i.e. into heresy.
The vowel kametz describes a true state of affairs, something objective, whereas the vowel patach describes something relative, a subjective truth. An example of the accuracy of this claim is 2Shmu'el 1:10, where the Amaleki lad relates to David that he had complied with Sha'ul's wish to give him the coup de grace so he would not fall into the hands of the Pelishtim as a prisoner, va-e'emod alav va'amottehu ki yadati ki lo yichye - so I stood over him and killed him for I knew that he would not live. In that verse the words va-e'emod alav - I stood over him, are an absolute truth; the words ki yadati ki lo yichye - for I knew he would not live, are a relative truth. The word va-e'emod begins with the vowel kametz under the letter vav, whereas the corresponding letter vav in the word va'amottehu, has the vowel patach to signify that this statement was only a relative truth. 1Shmu'el 31:4 states that Sha'ul had actually committed suicide when his arms-bearer refused to finish him off. So the statement of the Amaleki lad who claimed to have killed him was not an objective truth.
All the foregoing is part of the wisdom contained in the Holy Torah and the Holy Tongue. This is the reason that the relationship of consonants and vowels to one another has been compared to the relationship between body and soul as pointed out in the Sefer HaBahir by Rabbi Nechunyah ben Hakana 1. The vowels, i.e. the absence of correct vocalization invalidates the text just as the absence of a soul paralyses the body. The body is compared to an animal without a rider. The soul supplies the animal with its rider. In a similar manner the vowels are what complete the consonants. We find a statement in Megillah 3 that the words veshom sechel - and [they] gave the sense, in Nechemya 8:8 are a reference to what is spelled out in the Holy Scriptures, especially the vocalization.
The Kabbalistic approach to the vowel kametz in the word a-donai a combination of two of the Holy Names of G-d both comprising four letters. The one name symbolizes both G-d's preceding any phenomenon in the universe as well as His Oneness and uniqueness in the world. The second Name of G-d in that expression teaches the Nobility of G-d, that He transcends even the highest of the ten emanations. This is why the word a-donai commences with the letter alef and concludes yud. The letters dalet/nun in the middle represent the Attribute of Justice.
The three Names of G-d which are comprised of four letters each are all alluded to in a single verse in Shemot 35:17 Et kal'ei hechatzer et-amudav ve'et-adaneiha - the hangings of the court, its pillars, their sockets, represent the all encompassing Name of G-d, the Name E-hyeh which testifies to His being Eternal and unchanging. The word et-amudav represent the Ineffable Name YKVK; finally, the words ve'et-adaneiha are comprised of the letters in the word a-donai and symbolize His relationship as Master of the universe, i.e. the influence of what is above on what is below.