Can you triple-boot safely?

This is perfectly safe, but you want to know what you're doing. Keep in mind that you may only have four primary partitions on each disk. With one partition for XP, and three total for your Linux partitions (two regular partitions for the system and one swap partition), you have four partitions and won't be able to safely add any more. If you do wind up deciding to add more partitions, you may consider keeping a few or all of your partitions in an extended partition. An extended partition is only a primary partition that can be divided up into (or "contain") several partitions, up to 64 of them. You can create one with GParted, make your system partitions, and install your operating systems to there.

If you definitely do not want to ever add any more partitions to your hard disk (you only mentioned one) other than what you will have, then you don't want to mess with extended partitions. Keep in mind that if you move the Windows partition to any other position, for example making it be the second partition on your hard disk rather than the first, then you will need to edit your C:\NTLDR file to reflect the change.

I recommend installing Windows first, or keeping it on the disk if it is already installed. This will probably save you some trouble, because one often has to deal with difficulties with partitioning in Windows. Then, from Ubuntu or Backtrack live media, resize the Windows partition so you have enough room for both Backtrack and Ubuntu. I would allow 7-10 GB for Backtrack, depending on what you want to do with it, and at least 25 to 50 GB for Ubuntu. Leave additional space in your Windows partition, too, of course. Now you want to actually create an empty partition on the end of your hard disk for Backtrack, optionally inserting your extended partition beforehand, and then install Backtrack to it. The install Ubuntu, which will recognize your unallocated space during the installation and give you the option to use it, creating a swap partition and an EXT4 system partition within it. If you already have Ubuntu installed, then simply shorten the EXT4 partition and move the swap partition to the left, and then create you Backtrack partition on the end and install it.

Once finished, if you already had Ubuntu installed and only wanted to add Backtrack, you will probably want to run boot-repair. In Ubuntu, you will be updating your kernel frequently, or at least more frequently than you will be in Backtrack, and you want Ubuntu to handle the updating of the Grub menu and Grub software package. If you installed Backtrack first and then Ubuntu, then Grub will already be handled by Ubuntu and you will have no need to run boot-repair. In the end, you will have the option to boot any of the three operating systems you have installed, as well as memtest, upon PC startup.

I hope these suggestions and info helped (:

Yes! It's no harder than dual-booting. Of course, just like with dual-booting, you'll need to decide the partitioning and which bootloader you want to use, but that's all nothing new.

So do you already have it set up to dual-boot? If so, you just need to carve out some free space (from the Ubuntu installation, I'm guessing) to make room for Backtrack, and then go about the installation just as you normally would. I'd suggest that when you get to the screen asking where to install it, you select "Something else" and choose the empty space you created.