Windows installs its boot files in the "active" partition regardless of where you tell it to put the OS.
In a "normal" Windows multiboot (oldest OS first, newest last) that will mean that Vista adds its boot files to the XP partition and takes control. Subsequently, W7 will add its entry to the Vista BCD on XP, replace Vista's bootmgr with its own and take control in turn.
The advantage of this (and microsoft's design intention) is that you need do nothing. A multiboot happens automatically with no 3rd party intervention necessary.
The disadvantage is that the BCD is full of descriptions like "previous version of Windows" rather than a simple "XP", though EasyBCD will sort out those cosmetic details for you in a trice. Also, should you wish to remove XP (as you might in a few years when support ceases), there is no easy way to remove XP without crippling the boot. EasyBCD can also fix that for you
Changing the Boot Partition - EasyBCD - NeoSmart Technologies Wiki.
Windows 7, if left to its own devices in a single-boot situation will allocate the "system reserved" partition (100Mb) which it will set active and thus install its boot files in. There is absolutely no point to this unless you intend to encrypt the OS partition, in which case the boot files need to be outside the encryption zone.
You can see from the above that in order to get three
completely independent OSs it is only necessary to set the target partition "active" just before booting the installation CD/DVD.
That way you will have 3 primary partitions, each independently bootable (provided that you reset the active flag to the one you want to boot next before closing down the running OS) and no wasted 4th primary sitting with a couple of boot files in it.
After you install XP it will obviously just run itself. Adding Vista (in an active partition) will boot only Vista (being completely unaware of XP in the previous partition), but you can at that point run EasyBCD and add XP to the Vista BCD. You will have a working dual boot.
After adding W7 (to an active partition) it will boot alone being unaware of the other 2 systems. At that point you can add Vista/XP entries to its BCD, and it will be capable of triple-booting. This won't remove the ability to dual-boot Vista/XP by setting Vista active, nor the ability to boot XP by setting
it active.
This has the advantage that should you somehow break the boot, you can recover by booting another OS by the simple expedient of setting the active flag to a different OS.
Having completely independent OSs has the disadvantage that MS doesn't expect it, so installing a service pack or some other update which might affect the boot, might not be handled properly unless you've taken the precaution to set the system you're updating "active" before you start.
Take your choice !
Remember also that XP destroys Vista/7 restore points.
Run this hack on XP after installing Vista/7, but before starting to customize or use them.
System Restore Points - Stop XP Dual Boot Delete
