Gaming Rock Band questions...

[Gagarin]

If I have Rock Band for 360, Can I use the guitar for Guitar hero 3 on it?
Can I use ps3 controller on my 360? (it's USB)
My friend has Rock band but on ps3, is there any way to play with him online?

 

[Gagarin]

and one more. I my friend bought some songs from ps3 online service, is there any way to transort it on my xbox?

 

[Armadillo]

If I have Rock Band for 360, Can I use the guitar for Guitar hero 3 on it?
Can I use ps3 controller on my 360? (it's USB)
My friend has Rock band but on ps3, is there any way to play with him online?

Yes (Assuming you mean the guitar from the 360 version of it)
No
No

 

[hankchill]

and one more. I my friend bought some songs from ps3 online service, is there any way to transort it on my xbox?

No. If it's on PS3, you're stuck with it. Same goes with 360.