Well, TrueCrypt doesn't support system encryption on Unix/Linux systems. Now, that doesn't necessarily mean this isn't possible, but I think you might have difficulty. This guide shows how you can encrypt the system partition on a multiboot system, but it leaves the other systems unencrypted.
If all of your OS partitions are on the same drive, I think you could try encrypting the entire drive. Then, you would be prompted with the TrueCrypt loader at first boot. Once you entered that, it might be possible to set things up so that TrueCrypt then dumps you into GRUB where you can select which OS to run. You could try that by doing the following.
- Create all the partitions you need ahead of time (Win7-1, Win7-2, Linux-1)
- Clone your Win7-1 partition to Win7-2, using your choice of software
- Install your Linux distro of choice.
- Make sure GRUB is working and allows you to boot into all three OS.
- Boot into Win7-1 and use system encryption, selecting to encrypt the whole drive.
I'm not sure if that would work, but it might. Make a good backup of your main system OS before you get crazy. If the OS are installed on different drives, I would suggest that the easiest way to handle things would be to use system encryption on each of the Win7 drives, use a Linux-compatible encryption option (Ubuntu has FDE, for instance), and then set up your BIOS to prompt you to pick a drive to boot from, or something similar.