Professor cc to other professors when I ask about the PhD student vacancy. Should I answer as a group or to individuals? I wrote an email to Prof. X to ask about a PhD student vacancy and talk about a topic in one of his papers. He refused to accept me because of his retirement and replied with a warming email. In that, he cc-ed two others that he thought might be interested in me, Prof. Y and Prof. Z. Prof. Y refused because his area is somewhat distinct to mine; Prof. Z hasn't answered yet. I haven't responded to anyone yet. I also want to ask Prof. X's opinion about Prof. T's work, who is actually the one that I want to follow.

How should I respond to this? My feeling is that if Prof. X cc-ed others, this means that he wants to create a group discussion. As a group, everyone should have the right to hear each other voices. Should I reply to the group with a generic "thank you" email?

But if I do that, then I have to send another private emails to Prof. X and Prof. Z. I just worry that this will make me looks sneaky because I don't have the courage to discuss this in public. 

What should I do?
The key factor here is that the professor X is retiring.  He is not attempting to initiate a group discussion, he is trying to pass you off to the other CC professors and give you an introduction to them.

The correct way to respond is to send a nice single-addressee thank you note to original professor X only, perhaps also asking your query about Professor T, then send a single-addressee email to each individual Professor listed in the CC.

The key here is that you won't be working with the group, you'll only be working with one of them.