What is the origin of the Doctrine of Trinity?

My understanding is that "Trinity" is not is not used in the Christian Bible to define the three hypostases: God, Christ, and Holy Spirit. What then is the historical origin of the Doctrine of the Trinity? Put another way, when and from whom did the word "Trinity" come about in relation to the hypostases? Is there a reason "Trinity" appears in the Christian lexicon rather than referring to the specific hypostasis?