You should just be able to re-install Win 7 to the same partition it was already on. You'll want to format it (like sawdust showed), but the installer should pick up the other two Windows installations and add them to the bootloader automatically. (The Windows installer is pretty good about picking up other Windows installations...don't get me started on its habit of silently overwriting any other bootloader!)
Something semi-unrelated to the original question to keep in mind regarding sawdust's post: Windows does not always mark its boot drive as C:. I'm not totally sure under what cases it doesn't, but often if I install Windows to a partition on a hard drive that already has a Windows install on another partition, it will install itself to D: (or the first available letter).
I've had Win 7 installed for a while, and when I installed Win 8 on another partition, the Win 8 installer picked up the existing Win 7 install, and therefore installed itself to D:.
Again, I'm not totally sure exactly under what circumstances this will happen, but it can be confusing if you aren't expecting it!
Is EasyBCD easy to install and configure? What details would I need before hand to set it up? – orange – 2012-08-16T12:16:49.940
1As its name implies EasyBCD is not complicated. I mentioned it only for completeness as a possible recovery method for restoring triple-booting. The Win7 installer should be able to find the other Microsoft OS partitions. I know it can find an XP partition, and automatically add it to the boot list. – sawdust – 2012-08-16T20:19:14.337