First, I would suggest that all your sentences should be punctuated properly, otherwise it would cause ambiguities and may lead to misunderstandings.
1. 好像你的角色就定了这个，有什么好笑的? (your sentence doesn't make any sense to me, which would sound good to me with minor changes)
It would appear that it's been decided you're gonna play this role. is that funny?
2. 和音乐老师学了吹这个了吗? 当然了
Have you learned how to blow this (instrument) from your music teacher? Of course! Yes!
3. 今天他大反转, 让我们看到了他真挚的一面
He became a completely different person today, letting us see another side of him -"sincerty".