Dm3 is a perk in the custom Nazi Zombies maps Tot Schule, Ice Breaker, Quarantinae, and Insel der Toten. This perk allows you to have a Death Machine during approximately 4 to 5 rounds and cost 3000 points (which is quite expensive).
It can be quite useful when your ammo is low and changing your guns is not an option. The problem though is that the death machine powerup causes you to walk slower - you are unable to run without Stamin-Up. And even with Stamin-Up, you will still run at a slow pace.
You are still able to revive players, throw grenades, and buy perks. If you wish, you can upgrade your minigun. This will give it more damage and an (even) faster firing rate, but it will still go away once your time is up. The upgraded Death Machine is called the V-2 Death Machine.