The number 3 and the term triad are prominent in ancient pagan philosophy and religions.

Aristotle, in De Caelo says:

“For, as the Pythagoreans say, the world and all that is in it is determined by the number three, since the beginning, middle and end give the number of an ‘all,’ and the number they give is the triad. And so, having taken these three from nature as (so to speak) laws of it, we make further use of the number three in the worship of the Gods.”

Most, if not all, Church Fathers were pagan philosophers and very familiar with Egyptian and Greco-Roman pagan lore.

Furthermore, throughout antiquity triple deities and 3-in-1 deities abounded!

Ancient Egypt:

• Ptah-Seker-Ausar: the triune God of the Resurrection. (The Book of the Dead) 1550 BC.
• “I am ‘life,’ Temu, the lord of years, a lord of eternity. Shu and Tefnut were brought forth from me. Thus, from being One God I became Three.” Pyramid Texts, c. 3000 BC.
• “One is Bait, one is Hathor, one is Akori, to these belongs one power. Hail, Father of the world! Hainl, God in three forms!” Text in Greek on Egyptian Amulet, 2nd c. BC.

Greco-Roman:

• “Hermes [a gladiator], alone is all and three in one.” Martial, Epigram 5.24, 1st c. AD.
• “Where is Geryon, the three-times one?” Tertullian, De Pallio 4.3, c. 3rd c. AD. NOTE: The labors of Hercules included the capture cattle of Geryon and Cerberus.
• “One is Zeus, one is Hades, one is Helius Serapis.” Julian, Orations 4, c. 4th c. AD.

For more: https://en.wikipedia.org/wiki/Triple_deity