Orthodox Christian theologians define the doctrine of the Trinity is that the Father is God, the Son is God, the Holy Spirit is God, and together these 'three Persons' make one God; and these three are co-equal and co-eternal, the Son having been 'eternally begotten' of the Father, and Jesus being simultaneously 100% God and 100% man.
Today, many Christians think they are Trinitarians simply because they believe in the Father, Son, and the Holy Spirit - but that is not the Trinity!