I've installed Windows with the Boot Camp Assistant Tool but I want to create a third parition to install Ubuntu. On the Mac I can't change the volume size, is not that i receive some error, is just don't give me the ability to shrink it. If i try to resize the Windows partition from Windows (so to get some free space from there), Windows says that the maximum aviable space that can be reduced is 600 MB (even if I have 30 GB free on Bootcamp partition). Is there a way to create another partition without deleting Bootcamp?
Winclone is the OS X software to backup and restore the Windows OS on a BootCamp partition. http://twocanoes.com/winclone/
It lets you escape Apple's restriction to only have two OS partitions on a drive. You still might have to delete bootcamp, but you can then restore it in working form after you've re-done the boot drive partitioning to your liking.
Also see this question for core storage complications:
You'll need a third party utility that can repartition a live drive. The Apple software will not make additional partitions beyond the Bootcamp one. Or, the hard-but-effective way is to back everything up, wipe and repartition, then copy back. Winclone is a good program for backing up Boot Camp partitions.
Personally, this issue was worth it to me to buy Parallels and run Ubuntu (and eventually Windows) on a virtual drive. That way it's just files on my single Mac partition.