Here is a 13th century fresco from the Djurdjevi Stupovi Monastery (Đurđevi stupovi) in Serbia:
It depicts three angels seated a a table. You may recall that we have seen this basic image before, as part of more detailed icons of the “Old Testament Trinity,” also called “The Hospitality of Abraham” (Гостеприимство Авраамово/Gostepriimstvo Avraamovo). But in this example, Abraham and everything else found in the more elaborate images of the Old Testament Trinity is absent.
We can see the title inscription written at both sides of the halo of the central angel:
СТА ТРО ИЦА
From past postings here, you will recognize that curved line above the first three letters as the sign of abbreviation, and so you will know that СТА with that line above it is the abbreviation for СВЯТАЯ — SVYATAYA — meaning “Holy,” except of course that the third and last letters would be given in the old Slavic form instead of the later Russian Я form for the “ya” sound.
You will probably also easily recognize the separated ТРО ИЦА as the word ТРОИЦА — TROITSA — meaning “Trinity.” So this image is titled Svyataya Troitsa — “Holy Trinity.” You likely also easily noticed that the letters ТР (“TR”) are linked — they form a ligature, with the curve of the Р (“R”) about halfway down the vertical line from the top bar of the T.
Now the second significant thing to note is that this same central angel has the cross in his halo. I hope you remember that a halo with a cross in it is used for images of Jesus. Here that means the central angel of this Trinity is identified as Jesus — the Son, and the other two angels would then be the Father and the Holy Spirit.
One of those inter-factional quibbles eventually arose over giving the central angel the cross halo, thus identifying him as Jesus. The Russian Stoglav Council in 1551 forbade this practice, and decreed that the title of such icons should be Svyataya Troitsa, and should follow the model of Andrey Rublev; and even today Eastern Orthodox bicker over whether the practice of giving the central angel the cross halo is “uncanonical” or not. That does not matter to students of art history, however, because we are more interested in what was actually done than in what various factions think should have been done. In any case, the painter of this image had not yet adopted the then relatively recent (in Slavic lands) practice of placing the Ho On — “The One Who Is” — inscription that also eventually became characteristic of the halo of Jesus. He just used the simple form with no inscription on the cross in the halo.
Interestingly, there was not always agreement in earlier Christianity as to who the three angels were. Procopius of Gaza (465–528) wrote that some considered them to be three angels, while the “Judaizers” held that only one of them was God, and the other two were angels; still others considered the three angels a “type” for the Holy Trinity (See his Commentary on Genesis XVIII).
Justin Martyr, in his Dialogue with Trypho LVI, asserted that one of the three was God, and the other two angels, while his opposition held a common Jewish view that the “three men” of the Genesis story were three angels, and that they appeared to Abraham only after God appeared to him, following the text of Genesis 18:1-2:
“And the Lord appeared to him in the plains of Mamre: and he sat in the tent door in the heat of the day; And he lifted up his eyes and looked, and, lo, three men stood by him…”
The consensus in Eastern Orthodoxy, however, came to be that the three angels were the Trinity — or more vaguely, that the three show the “relationship” among the persons of the Trinity.