The giant robot that Dr. Wily pilots in Mega Man 3 is Gamma. Dr. Wily claims to have reformed and works with Dr. Light to build Gamma as a peace-keeping robot in the 21st century.
Gamma requires eight power crystals, which are guarded by the game’s eight Robot Masters. After those robots go berserk and take the crystals, Mega Man must defeat them and recover the stolen power sources. The Robot Masters include Magnet Man, Hard Man, Top Man, Shadow Man, Spark Man, Snake Man, Gemini Man, and Needle Man.
Once the crystals are recovered, Wily betrays Dr. Light, steals Gamma, and takes the robot to his fortress. Gamma becomes the final major machine Mega Man confronts, although the battle ends with Mega Man destroying it. Players sometimes confuse Gamma with the Wily Machine or Wily Capsule, which are recurring fortress bosses in other Mega Man games. Gamma is instead the peace-keeping robot central to Mega Man 3’s plot.