"Complete wave 3" is an objective in the sidequest Hyperion Slaughter: Round 2 in Borderlands 2
Walkthrough
This third wave has a few PWR Loaders coming at you from the South. They are quickly backed up with WAR Loaders. There are only a few of them for you to deal with initially but the WAR Loaders make things a lot harder to deal with. Get through them and you have one wave left.
