I am trying to triple boot windows xp with a pc that already has 10 and 7 installed.
I managed to install XP on a partition. It then overwrote the MBR. I accidently erased the MBR and reset it for 7 which also reset 10. Is there any way I could use all 3
I thought you broke the boot partition for 7 and 10. If you still have access to those, you can manually change the boot menu from windows. This should get you started.
I have managed to create a boot entry for XP in Command Prompt. It shows up on Easy BCD and command prompt but not on msconfig which means it is not recognised on boot.
You might want to let a Linux based "Boot Repair Disk" sort it out for you... it will find all the OS's on a machine and create a boot login menu. (grub)