1.You'd better put on your coat,____?
2.You must go back right now,____?
3.He must be in the classroom,____?
我先谢谢各位啦!

陈述部分有had better + v. 疑问句部分用hadn't you? 例如：
You'd better read it by yourself, hadn't you? 你最好自己去读，好吗？
陈述部分有must 的疑问句，疑问部分根据实际情况而定。例如：
He must be a doctor, isn't he? 他肯定是医生，是吗？
You must have studied English for three years, haven't you? / didn't you? 你一定读过三年英语，对吗？
He must have finished it yesterday, didn't he? 他肯定是在昨天完成任务的，是吗？
must在表"推测"时，根据其推测的情况来确定反意疑问句。例如：
He must be there now, isn't he? 他现在肯定在那儿，是吧？
It must be going to rain tomorrow, won't it? 明天肯定要下雨，是吗？
所以答案应该为:
1.hadn't you?
2.won't you?
3.isn't he?