What is the easiest and safest way to perform triple-boot?
Solution 1:
There's no difference between dual-booting and triple- (or quadruple- etc.) booting, at least when the OS being installed is Linux - you just prepare an empty partition and install your OS there.
Supposedly, the GRUB will be re-installed so its second stage will be on the partition of the last installed OS, but it should find all other OSes and add them to the menu just the same.
I'm not sure how repairing MBR with Windows DVD relates here though - you aren't using Windows bootloader as your primary bootloader, are you?