Trinitarians! Does Joel Osteen believe in the Trinity ?

According to Joel Osteen Ministry's statement of faith, yes.

He is off track in many ways. One is teaching that we can have our "best life now" as if this pig-sty of sin and death and lying and treachery is better than heaven. "Oh, praise be, I got a new car. It's the BEST thing I could ever have."  "Harrumph," says God.

Many are jealous. Others disagree with his particular "take" on things. Especially the emphasis on prosperity and material goods.

Yes. Many people don't like him because he preaches a word of faith/prosperity doctrine.

Joel Osteen does not disclose what he believes because he is a scam artist who makes millions telling people that God loves them no matter what.  You can have and abortion, murder, adultery and even steal stuff just send me enough money and God will love you.  But he is a politician who has learned to never state what he believes, wether he believes in hell fire?  Trinity or even that Jesus was God or not.  He has learned to talk and not say anything.

there are so any reasons to loathe Osteen that his theology, fake or sincere, doesn't even make the top 10 list. 

Trinity: I have no idea (because I am not a follower or fan of Joel Osteen).

Don't like: because of certain doctrines that he DOES teach

typically: the latest fad doctrine

such as the so-called "Prosperity Gospel" (which I like to call "Jesus for Money").