I Dare say that most religions that call them selves Christian believe that God, Christ and the holy spirit are all one. It is called the trinity. Are you saying that those that believe in the trinity are not Christian? Or are you saying that Christians are polytheist?
The usual term of derision is "triune god." I've seen it used by proselytizing atheists, and by ultra-Orthodox Jews. Such is the nature of religious freedom. You take the good with the bad, as far as speech. No one is obliged to speak well of my religion, just as I am not obliged to speak well of anyone else's.