I now have XP and Ubuntu dual boot (using Grub), and I have to add Vista. Google gives a nice guide for doing this, but it assumes I'd only have one hard drive or would like to make a partition from drive XP is on. I, however, have several and would like to keep OS's apart.

Now, would this work?
1. I add and format a new hard disk using XP. Shutdown.
2. I detach all other drives but the new one and DVD-drive. I set boot priority for DVD.
3. Install Vista on the new drive.
4. Shutdown after installation and attach XP and Linux hard drives, set Linux HD as first priority boot drive.
5. I have triple boot or still XP/Ubuntu dual boot?

You shouldn't have to detach any drives, as long as you are comfortable working with them in a limited graphical view.
Though, this could cause an issue with vista, I don't know how it writes to the MBR, so it would actually be safer to set the new Vista drive as the primary drive and install like normal.

Once you have vista on your 3rd hdd, connect everything back up, with Ubuntu as your primary drive and then add the boot-loader information to grub, and make a menu option for it.

At this point, you should be able to boot to all 3 OS's. If you have any questions, pm me.

XP, Vista & many linux distro's are easy to set up together... even osx if you feel so inclined.

Easiest way is to have everything attached from the get go...

All hard drives, if you are not partitioning one.
All DVD/CD, etc.

Example: Space 1 = first hard drive or partition (XP)
Space 2 = second hard drive or partition (Vista)
Space 3 = third hard drive or partition (Linux)

Start by installing xp on space 1. Then, install Vista on space 2. At this point, vista bootloader will recognize the xp install and add it as an option when booting.

Then install any version of linux in space 3, grub bootloader preferred. This will change the first bootloader to grub which will give you the option of booting linux OR Windows. If you pick windows it will load the vista bootloader which lets you select xp or vista. Easy!

Just give vista the total boot. Get rid of it. With the other systems why would you even want such a problem riddled system as vista