*The simple way, create a new virtual machine and have a completely virtual Ubuntu machine.
*The hard way, it will let you boot natively into any of the three OSes, as for Parallels, you'll have to edit the .pvs files so it recognizes the proper Boot Camp partition.
Triple Boot: http://wiki.onmac.net/index.php/Triple_Boot_via_BootCamp (don't know if it's the most up to date article but it's just an example, Google is your friend and the keywords are: "Boot Camp" and "Triple Boot").
If this information is not enough to get you going, and you need further assistance for triple booting, then forget it, triple boot is not for you, triple booting requires some level of knowledge beforehand.
Note: I don't think Boot Camp Linux is supported in Parallels, so you can't probably boot as a virtual machine a Boot Camped Linux installation, never tried it, althought it would be an interesting experiment.
Last edited: Aug 25, 2008