It would be best to install Win7 first. by going with Linux first you will get the GRUB loader by which will then be replaced again by the BCD once you install Win7. Since the BCD doesnt have default settings to include Linux or GRUB you would then lose the ability to boot to that partition till you used EasyBCD to add it back.
If you go with Linux after you will then have the BCD set for the dual boot with Vista/Win7 and then GRUB would be installed and you will have your triple boot set easily.
Of course if you want to just use the BCD you can install Linux now and then win7 later and do what i said above. Use EasyBCD to add a entry for Linux and have all 3.