Someone who believes in Jesus Christ (and the Trinity) is a Christian.You have 3 different issues there. I will deal only with this one.
MOST (not all) Messianic Jews believe in the Trinity.
Are you saying they (in your opinion) are no longer Jews? On what basis?
When I speak of Jews, I am speaking of those who are adherents of the religion of Judaism. In Judaism, the thought of God being Three Persons is anathema.