Jeter:
I have no idea about what he would be like as a first basemen, so lets look at what we do know. When compared to the other starting shortstops in baseball, he's near the bottom. He's also not the best shortstop on his own team. It is also more important to have terrific defense at shortstop as opposed to terrific defense at first base. Because of all those factors, I think they would be better off if he changed positions. I'm not saying it's going to happen in 2008, but before his Jeter's contract runs out (2010), I would not be surprised if he wasn't playing shortstop at some point.
Santana:
I completely understand you not wanting to part with any of those three, and as you said, I think a lot of people (including some in the Yankee's organization) would agree with you. However, if you want Santana, I firmly believe you're going to have to give up one, and probably two of those guys. After what Chamberlain did last year, I don't think you can trade him, so that leaves you with Hughes and Cano. It's going to be difficult to sell a lot of people on this trade (including you apparently), so at this point I'm not going to even bother with that. What I will say is this...they've been eliminated from the postseason the past seven years, the majority of those years in ugly fashion. They're clearly missing something, and in my opinion that's a bonafide ace. Phil Hughes could very well develop into that, but we really don't know. With Santana we do, and if the Yankees can lock him up to an extension then they're doing much more than just replacing Phil Hughes.
As for Cano, that would be a huge loss, I'm not going to lie, so I think you need to look more at the gain rather than the loss. Cano is not going to be easily replaced, but you can to a lesser extent make up for his absence.