I know many people who question Catholicism.
It's not that they don't want to believe, per se, but it is that they need more evidence. Or explanation.
The Trinity comes up in almost all of those conversations.
"How can one Person be three People?" they ask.
First I correct them, and explain that God is not a person like us. It is important that they understand that, first and foremost. Yes, Jesus came to us in human form, but certainly God is something much more Spiritual than us. That alone should help us realize that the Trinity is beyond Human example.
Then I try to use something earthly to help them along. Take water for example. It can take the form of liquid, ice or steam. Three different forms, but still H2O at its core. Most people usually get this, but can't make the leap from water to God.
Then I usually end the conversation by saying that I don't fully understand the Trinity either.
This is usually received with confusion.
I explain that it is impossible for us to fully understand the Trinity, and many other Mysteries. We can only understand up to a point, and the delta between that point, and the full understanding, is made up by Faith.
The difference between them and me, I explain, is not that I understand God's mysteries any more than they do. The difference is the amount of Faith we have.
And then I say a silent prayer to the Holy Spirit to strengthen theirs.