Hi Joce,
Note - XP cannot be booted from UEFI.
You can triple boot XP + Win7 + Win8 only on BIOS firmware and MBR style disks.
I assume you have either one MBR disk with W7+XP and add Win8 to a free partition
or add another disk(again MBR disk!) with Windows 8.
If you install Windows 8.1 after having Windows 7 and XP it will pick up the current setup,
install itself as default OS and on boot you would have three boot choices in menu:
Windows 8, Windows 7 and XP.
---------------------------
If you have another disk with Windows 8 and want to bind it to existing boot menu(W7+XP)
you have to do the following:
On BIOS - active partition, first disk should be updated with Windows 8 boot manager
and an entry for Windows 8 should be added to Windows BCD (on active partition, first disk)
I think that as well as Dual-boot Repair in 0.9.3.1 as well as the new version could do this.
Dual-boot Repair/Automatic Repair in your case this would add Windows 8 to existing boot menu(W7+XP)
and update boot manager.
All the best
Bo