They do not believe that God is three in one. They believes that God, being omnipotent, can appear to humans in many forms, including becoming human like Jesus.
Is that avodah zarah too?
On another edge, Christianity also have Unitarianism which I think have the same concept of God with judaism. In this question, I am asking for Sabellianism, which is somewhere between Unitarianism and trinity "flavor" of Christianity.