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http://en.wikipedia.org/wiki/Sabellianism
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.
The Rambam states that believing G-d has a body makes one a Min (a heretic), all the more so saying G-d was incarnated.
As for Unitarianism, it would probably be like Islam, which believes in false prophets rather than being outright Avoda Zara.
Of course, in either case, it would be forbidden for a Jew to adopt Unitarianism or Islam; but would not have to die rather than do so like outright Avoda Zara. (i.e. under duress, it would be permitted, according to the Rambam, though not all authorities agree with this since a few say Islam is also Avodah Zara, to say "I am a Muslim" in order to save one's life, but not so in the case of believing in the incarnation.)
