think to be, consider to be, regard to be 这三个词组中哪个为对的？

都是对的。
例如：
We think the question to be easy.
We consider the question to be easy.
We regard the question to be easy.

如果用被动语态，也都是对的。
例如：
The question is thought to be easy.
The question is considered to be easy.
The question is regarded to be easy.

并且，你还可以把to be省略掉，变成:
We think the question easy.
We consider the question easy.
We regard the question easy.
The question is thought easy.
The question is considered easy.
The question is regarded easy.

consider to be是正确的.另外consider to be中to be可省略,或用as. regard… as的意思与之差不多.