I have been holdin back on guitar hero 3 because i wanted to know can u play the game if your left handed?

When your left handed u hold the guitar the other wat around so is their a feature that allows you to play left handed?

yes....in the control options you can "lefty flip" the controls so you can hold it the other direction.

I don't know about Guitar Hero III, but I know 1 and 2 have the lefty flip option so I assume 3 does too.

To be honest though, I just play normally.

I believe there is an option to switch all the notes to other side.. an left-handed option. I'm about 90% certain

My sister is left handed, but she play like right handed, and yes, there is an option for change that

Is it the same for Rock Band???

All the Guitar Hero games as well as Rock Band feature this option.  Rock Band helpfully informs you that such musicians as Kurt Cobain and Jimi Hendrix would have made use of it.

Why would it matter if your left handed, it would still work the same.

Try playing the game with the notes on the screen being displayed in the opposite formation from the frets on the guitar.  It's not so easy, believe me.

I dont know.. Ive never tried it myself.