
They understood it from the scripture.
Yes, because, like all good Catholics, they read the scriptures in the light of the Apostolic Teaching.
That doesn’t prove that the teaching of the Trinity isnt revealed in the scripture, and it doesn’t prove that the early church didn’t discern the Trinity from the Scripture.
I think that scripture can “prove” very little. It represents the doctrine of our faith, and it is easy to “prove” errors with it, as many heretics have done. The Arians and the Gnostics also used the same scripture.
We are in agreement that the mystery of the Trinity is revealed in scripture. However, it is not visible to those who approach it in doubt. The Fathers did not need to “discern” this Teaching from scripture, since they had received it in it’s fullness from the successors of the Apostles. They could confirm what they believed from the Scripture because they read it in the light of the faith. This has always been the proper use of scripture.
Luke 1:1-4
1:1 Inasmuch as many have undertaken to compile a narrative of the things which have been accomplished among us, 2 just as they were delivered to us by those who from the beginning were eyewitnesses and ministers of the word, 3 it seemed good to me also, having followed all things closely for some time past, to write an orderly account for you, most excellent The-oph’ilus, 4 **that you may know the truth concerning the things of which you have been informed. **
The scripture supports the Apostolic Teaching from whence it came.

They had no place else from which to understand the Trinity. Read their statements concerning it. The Trinity not merely your tradition, it’s revealed in the scripture.
I think you must cling to the idea that “they had no place else” in order to reinforce your SS doctrine. You are wrong, however, they had the Teaching of the Apostles.
I agree, the doctrine of the Trinity is not merely found in the Sacred Tradition, but it is also reflected in the Scriptures.