An easy way to salvage enemy mechs is to shoot off one of their legs. Have a group of mechs and concentrate your pulse lasers at one leg. This increase your chances for success.
Use explorer on your Mechwarrior 3 CD. Then open the folder named "video". Double click on any video you want.
Unlimited Ammo and Invincibility
Beat Instant Action and you get Unlimited Ammo and Invincibility but these cheats don't work in the campaign.