The three dead Grunts, with the three Plasma Pistols in the background.
The Three Dead Unggoy is an Easter Egg in Halo PC and Halo: Combat Evolved.
They can be found at the beginning of the level Assault On The Control Room if you jump all the way down to the bottom of the pit. Since you will die from such a high jump, mods are required to view this Easter Egg. It consists of three Grunts (two Grunt Minors and one Grunt Major), laying together with most of their geometry overlapping. It should be noted that the Grunt Minors are "Curlback" Grunts.
These are, in fact, the Grunts from the previous cutscene. Scripts in HEK (Halo Editing Kit) will show that the Grunts in the cutscene die and are teleported to the pit. This being said, it is not a genuine Easter Egg.
You can also view the Easter Egg by starting the level with two players, and having one player exit the Pelican while in free-fall. This is, however, difficult to pull off.