It is possible. I assume you are already dual booting with XP on one partition and Vista on the second, while the 3rd is empty waiting for W7 to be installed.
1) Go to BIOS and set to boot from CD-ROM
2) Put Windows 7 disc and boot from it
3) When screen says press a key to boot from CD-ROM press a key
4) When the installation process asks you where to install W7 point it to the 3rd (empty) partition.
5) Follow the instructions and nothing else to do until let it finish.
Windows 7 will automatically detect the other 2 operating systems and will create a multiboot screen whenever you switch your computer on, that asks you which OS you want to boot into.
I also suggest you use EasyBCD after you boot into W7 to customize your boot menu even further if you want to.