Use Fedora (
www.fedora.redhat.com) it's a more updated version of CentOS with more packages available, Centos is based on RHEL and is designed for bussines critical Servers.
I have OS-X and Fedora Core 5 on this laptop (Mac Book Pro).
My Desktop pc has Fedora Core 6 and Win XP pro.
I've used Ubuntu in the past, it's a good distro if you're a newbie to Linux. But you'll get frustrated if you try to more complex stuff.
As for triple booting there is not all that much point. OS-X "under the hood" is fairly similar to linux (Bare in mind its not free and it's not open source, so downloading it is illegal).
If you do end up using Linux make sure you familiarise yourself with the package management system your distro uses, package management is the single best thing about running linux, makes it trivial to install thousands of different programs at the click of a button.