According to the software engineers at OnMac.net, the Triple Boot procedure "chainloads" the Linux bootloader from Windows XP after Windows is installed on a Mac.
It's as complicated a process as it sounds, and OnMac.net warns that only advanced users should attempt the feat because it involves manually partitioning the hard drive, a task that could destroy all the drive's contents.
To complete the process, Mac users run Apple's Boot Camp utility to install Windows XP. After installing the appropriate software and configuring the installation, the process of loading Linux can take place. Full installation instructions can be found at OnMac.net's Triple Boot via Boot Camp page.
