My response is that there's no possible way you can KNOW this. It must be taken by faith. Not that there's anything wrong with that but then there are those who get offended and act surprised if, God forbid, somebody asks a question about it. Not saying this describes you but I thought this is the FR a place to discuss these kind of things. :-)
That's an interesting comment. All faith, though, will eventually be unnecessary, as it gives way to knowledge. And even before that happens, faith rests on somebody's knowledge: that is, you may not know something first-hand, but you believe somebody who does. As Josef Pieper says, "Faith is participation in the knowledge of a knower."
Come to think of it, most of what we think of as knowledge is actually faith. I have been to Paris, so I know it exists. I have never been to Moscow, but my husband has. I have seen the word Moscow on maps; I have read about Moscow inbooks and magazines; I can look up Moscow on the Internet. You could say that my "knowledge" of Moscow is part of my larger "knowledge" of geography, but in fact it is faith. I believe the people who made the books and maps; I believe my husband.
The vast majority of what we "know" even in this world and in this life, is actually faith.
As for the Trinity: I am part of a long chain of people whom I do not think are lunatics or liars, who trust in someone who was a "knower," and who proclaimed the Father, Son, and Holy Spirit because He knew it first-hand.
I personally trust the people who handed this on to me.
In the end, both faith and hope will be gone. I will know, even as I am known.