Yes.

I believe in God as three persons in one divine nature, as explained by the Holy Trinity.

This is a good illustration of the concept (this illustration popularized by Bl. Fulton Sheen), understandably a difficult one for people to grasp:

I am not going to get into an essay on apologetics here. I spent much of my youth reading books on apologetics, both for and against the existence of God, and could never find myself fully embracing an atheistic position. I eventually settled on some form of deistic belief before actually believing in God as told in Christianity. I more or less considered myself "culturally Christian" because I thought the institutions were good and worthwhile, but I didn't follow Christian precepts and, looking back, would not have liked myself to use that label despite being baptized with a Trinitarian formula when I was 2 years old. 

There is a quick proof in one of Peter Kreeft's books that mirrors my experience believing in God as the Holy Trinity (unfortunately, I don't have the book on me, but it goes something like this):

1. Does God exist? (I think Aquinas provides sufficient proof here in the Summa)

2. If so, is is a specific god or a pantheistic god?

3. If not pantheistic, is it one God or many Gods?

4. If it is one God, is it an Abrahamic God or some other monotheistic God?

5. If it is the Abrahamic God, was Jesus Christ a diabolical liar, a lunatic on the level of a man who claims to be a boiled egg, or was he God Incarnate?

It takes time, but through a combination of historical research, anthropology, and good, old-fashioned Theology, one can get to #4. Then theology and prayer gets one over #5.

Actually knowing and admitting that I had faith in God took several months of regular prayer. I recently told a friend in conversation that it's important to keep in mind that faith is a gift and a virtue, not entirely something that is earned through study. I had learned to pray the rosary with a recording that notes that the beginning three Hail Marys are "for increases in the gifts of the virtues of faith, hope, and charity." 

"Gifts" denotes that these virtues are given and that you can ask for them. 

(An important note that my Catechism priest made clear to me at the beginning of the program, too, is that one can pray for these gifts even if one does not have them yet. That was huge for me.)

"Virtues" denotes that, once given, you can practice them, get better at them, and grow in them.

Praying for the gift of faith was a huge part of my journey to believing in God the Father, Son, and Holy Spirit, especially over the last ~6-9 months as I dove deeper into Christianity.