Maybe this is dumb BUT where does Vincent Crabbe go by the 3rd movie? He seems to be replaced by some skinny actor. Please someone tell me. In fact fat Vincent Crabbe isn't seen past the 2nd movie.Where did he go?They talk about him in all 7 books.

The same actor, Jamie Waylett, plays Crabbe in movies 1-6, but he's not in the last two because of the marijuana incident and stealing during the London riots.

Maybe he just hit puberty so he seemed like two different people.

Puberty, probably. That's just a guess. By that point in the filming, the actors were starting to hit puberty. You can'ttell as much with Dan, as he's so short, but Rupert really starts getting tall at that point.
Please be aware that this is only an inference based on my own observations. It is equally likely that they used a different actor. Here's a hint: listen to Crabbe talk. Their voices don't change too much over the series. Between #2 and #3, you'd be able to tell if they used a different actor by the voice.
Also, Crabbe was never described as being fat in the books. He was large with long arms, probably very muscular. And we know he is dumb.