That works yeah. What OSs do you have? Anyway you must restore your MBR to the state it had when you installed your first OS. Some Linux distros (at least SuSE i know for sure) stores a copy of the original MBR, so you can restore it from control panel. Anyway if thats not the case look here
Then you use fdisk (make a boot disk first) and delete the partitions with you unwanted OSs on.
Thats why i posted a link, but i can explain briefly. The MBR or master boot record simply controls the startup of OSs. To take an example, if you have winxp and linux on your computer, LILO or GRUB (dont mind the names) will be added to the MBR to allow you to choose which OS to load. Since you got 3 you know how it works. Lets say you wanna remove Linux and keep xp. Then you will need to remove the LILO or GRUB boot loader from the master boot loader, if not your system will believe to have Linux still installed on your system when you boot your computer. You dont want that, so you must restore the MBR to boot only xp. How? Look in the link.