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Negative SMG3 is a powered-up version of SMG3 but in negative. He is a supercharged version of himself colored in a photo-negative color scheme, and is activated when he is extremely angry over something. In this form he has the power to fly and shoot electric bolt projectiles.
This form first made a minor appearance in this blooper, when SMG3 takes over the Castle and puts the gang in another black room.
This form is what SMG3 turns into when he gets angry, because he lost a contest in this blooper. He is then defeated by SMG4 shortly thereafter.