Full list of all 38 Orcs Must Die! 3 achievements. It takes around 25-30 hours to unlock all of the achievements in the base game on Windows.
The base game contains 26 achievements, and there are 2 DLC packs containing 12 achievements.
Beat all "Cold as Eyes" scenarios
Kill 250 Cyclopes
Purchase all 3 pieces of "Cold as Eyes" gear
5 Skull one "Cold as Eyes" scenario on Endless
Kill 1000 Cyclopes
5 Skull all "Cold as Eyes" scenarios on Rift Lord
Beat all "Tipping the Scales" scenarios
Kill 250 Tuatara
Purchase one upgrade for any "Tipping the Scales" gear
5 Skull one "Tipping the Scales" scenario on Endless
Kill 1000 Tuatara
5 Skull all "Tipping the Scales" scenarios on Rift Lord