Aside from earlier reports, it apparently is possible to bring a third OS to the party, as OnMac.net has posted a how-to wiki for booting WinXP, Mac OS X and Linux on a MacBook Pro via Boot Camp (so far, this is untested on other Intel Macs).
Gentoo is the version of Linux used in the how-to, but the Slashdot post where I found this is confident that it would be simple to get other flavors of Linux working.
But forget all this triple booting stuff... when is someone going to crack the 4 OS barrier? I want a quad boot with Microsoft Bob running on a MacBook Pro!
Microsoft Windows 10