Is it possible to use two guitars and a keyboard at the same time?

I'm having the same problem not being able to use 2 guitars and the keyboard, so I'm guessing no v_v

If you mean guitar + bass + keyboard, I think you have to open up your menu, hit options, then click "All Instruments Mode." The only problem with this is that if you don't have a singer, the computer will "take over" on auto, and tends to do pretty terrible.

Hopefully that helps.

As an addendum, apparently you can only have 4 totals players (because of XBOX360 and Wii), so if you choose "All Instruments Mode," only 4 are scored (i.e. guitars + keyboard + drums = vocals aren't scored).

It's weird, but if you're using four instruments, vocals CAN'T be one of them. I think it's a bug, but you can only have four players if you're using two guitars, drums, and keyboards.

I think you have to open all instruments mode but you can only do that locally I think it won't work over xbox live or online play with wii and the ps3 at least that what seen so far i think though if you have a mic plugged in you can still sing the song with out the controller pluged it its just not scored but i have to test that out first

Basically, the Xbox 360 can only recognize 4 inputs, which is why the power button is separated into quadrants. This is a hardware limitation that only the Xbox 360 has, since it does not manage control inputs via bluetooth.
You have to enable "All Instruments Mode" on the options menu to allow for the playing of every instrument simultaneously.

Scoring and achievements will still count other than vocals.

It's even more annoying than that -- Last night, we tried to use mic + 2 guitars + keys...and it *forced* us into All Instruments mode (a screen popped up saying "do you want to enable AIM?" If we said no, only 3 inputs would be recognized). It made us sign out the mic user, even tho there were only four inputs connected *including* the mic user's controller. I hope that's really just a bug they need to work out, as it's exceedingly annoying.