Are you able to shrink Windows 7 or otherwise arrange partitioning to fit XP and Vista onto the HD? Post a screenshot up of your Disk Management drive map if you need help or advice.
When you install XP (first) it will take over the bootloader so you'll need to install Net Framework 2.0 and then EasyBCD 2.0 beta
(after doing quick forum registration to use beta) to Add Windows 7 back from XP.
It may be necessary to start Windows 7 to Add XP from 7 since EasyBCD works better in 7. To do this mark Windows 7 active in XP Disk Management, boot into Windows 7 DVD Repair console or Repair CD, click through to Recovery Tools list to run Startup Repair up to 3 separate times with reboots to start Windows 7. Now add XP in Easy Add/Remove tab: accepted offered boot files, autocompletes, Save, Restart. System Repair Disc - Create - Windows 7 Forums.URL
Installing Vista last may correctly configure the Multi-boot. If not, install EasyBCD again to Add Xp and Windows 7.
If you need help along the way, post back screenshots of your full Disk Management drive map and listings, and the Easy listings page.