Assuming your iTunes folder is under the Music folder in your home directory, I'd use an app called Synk. It synchronizes or backs up a folder of your choice. I do this with my music, pics, docs, safari and mail.
It works great and is easy and free.
Tho I gotta ask..why do you need to duplicate your files in 3 places?
I have a G5 2.0, an iBook 700 and a 15G 3G iPod.
I have all my music on a Firewire HD, hooked up to my G5 with my iTunes Library pointed at the FW HD. I also then mount the FW HD on my iBook when I need to access my music from somewhere on my AirPort network and I also have my iPod to use everywhere else. With an iPod, I never see a need to dupe my music collection. That's alot of GBs wasted.
Just my 0.02 cents...
macboyX