First off, the term trinity is man’s attempt to describe something very complicated, and no doubt we’re a bit off. None of our fancy metaphors perfectly represent it, and no one can clearly define it. The best we can come up with is probably still a fair way south.
Yes, Christians believe that God is one. He has always been and always will be. However, God has three “persons”: the Father, the Son, and the Holy Spirit (Matthew 28:19). Each of these persons is holy, eternal, almighty and each God yet at the same time of one God. Though the word trinity does not actually appear in the Bible, the concept fills it from cover to cover. Whether it’s concerning the deity of God the Father (1 Peter 1:2), Jesus (Colossians 2:9), or the Holy Spirit (John 14:16-17) it’s all in there. Messing with your head yet?
Don’t freak out. It’s ok. You’ll probably never completely understand it. And remember, it’s not knowing about God that’s important, but knowing God. Besides, isn’t there beauty in mystery?