Hi Kenneth, welcome to the forum.
If you install XP first, and then Vista; being backward compatible Vista should manage the dual boot automatically for you. EasyBCD should not be a necessary part of the process but, as its name suggests, is much easier than using MS BCD editor to tidy up, if you wish to.
Bear in mind that XP is not forward compatible, and will screw-up Vista's recovery points and shadow copies unless you take action to hide Vista from XP.
Most recent 3rd party apps put a lot of execution-time info in the registry, so will need to be installed on each of the systems. (you can search your current registry for the apps in question to see if this is so).
Some older apps only put uninstaller info in the registry, so can be executed from the other system (if it's visible).
I, for instance, have a few old apps which won't install on Vista (x64) because they use a 16 bit installer, but are installed on my 32bit XP and can be executed from the Vista system.
If you have 3rd party apps installed on Vista, residing on your third disk, the partition they're in will also need to hidden from XP to protect its restore folder.
Have a good read of the wiki about dual booting
start here
http://neosmart.net/wiki/display/EBCD/Windows+XP
and also have a read of the "Vista Hide 'n Seek" sticky thread in the "Ideas and Wishlist" forum, if you need to hide Vista from XP (Do you use system recovery ?)
When you've had a good look around the wiki and relevant threads, post back if you have specific questions about anything you don't understand.
Good luck.
Addendum:
PS
Also if one of your systems is D:\ and can see a C:\ disk, applications on D might put some information on the C disk (even if you tell it not to - Adobe reader for example), and the execution of this software might be unpredictable.
If Vista is C and hidden, and XP is D (like my system) then you won't have a problem, or if each system thinks of itself as C: and the other as D, there won't be a problem.
You will have to control the way the letters are allocated by the sequence and manner of the way you install the 2 systems. Have a think before you start about what you'd like them to be, and we can advise you how to achieve it. (If you go ahead without planning it - you're stuck with whatever happens even if it causes problems)