A trinity is any group of three things that are relevant to each other. This applies to a triangle, a three leaf clover, or a pizza cut into three pieces. Synonyms of trinity include trio, triad, and triplet. You can guess that if a word starts with “tri”, it has something to do with the number three.
Most of the time that someone talks about a trinity, they are referring to the “Holy Trinity”. In Christianity, the three parts of God are referred to as the Father, Son, and Holy Spirit. This is one of the teachings of Christianity that I never saw the relevance of. God could have three parts, but it is also possible that there is a fourth part of God that is named Bob. It is not possible to prove or disprove this theory but anyone is free to believe or disbelieve in Bob as they choose.
In case anyone asks why they have never heard of Bob, my guess is that Bob is an introvert who is not interested in showing himself to the rest of the universe. I am 99% sure that I created Bob in my own image, but since I can never be absolutely sure of this, I will not waste my time debating with anybody about the existence of Bob.