Question

they have three green T-shirts. 改为一般疑问句？

Answer 1

一般疑问句：Do they have three green T-shirts?
陈述句 they have three green T-shirts. 其中主语是they，动词是have，have是实义动词，不能提前，需要加助动词，因为主语是they，加的助动词为do，即疑问句中将Do提前就可以组成疑问句了。
望采纳。

Answer 2

They have three green T-shirts.
一般疑问句：
- Do they have three green T-shirts?
- Yes, they do.
- No， they don’t.