Yes, it is possible for you to do this. I would like to know have you installed all three OS's on the drives yet, or are you waiting to see if it is possible? The reason that I ask is that there can be some difficulties with installing a windows OS next to a linux OS but we can jump that hurtle when we get to it.
You can also load ubuntu on a virtual machine or simply boot live even from a usb flash drive without touching the Windows boot loader. There are various ways to set triple boots, quadruple boots even across multiple drives.
With SUN's Virtual Box you can run a distro on a virtual hard drive while booted in Windows 7 easy enough on the same drive.
That shows the 64bit Release Candidate on one vhd with 3 distros on 3 more on just one hard drive.
XP, 7, and Ubuntu all use different boot loaders. And each OS installation believes theirs is the best. You can get it to work though, just sometimes it's a pain to deal with. Virtual machines are your friends.
There's one free program that will actually add XP and ubuntu into the 7 BCD store you may have heard of called EasyBCD. Originally written for Vista you can use that program's form of NeoGrub to load Linux.
The tip for seeing a dstro added in as an option is not to allow Grub to replace the Windows mbr there. You custom install the intended distro and when prompted make sure the root partition(Linux system files) is labeled the "/" mount point making sure to check off the "do not install Grub into bootsector" box.
The one thing to note about EasyBCD however is that once installed and you start configuring the other OSs the new 7 look for a boot splash screen will be replaced with the one for Vista. It does just as well on 7 as found out on previous occasions while testing the 7 beta builds but still has that one drawback and why you may prefer the virtual machine idea better!