Lefty, Righty, and Slim

Lefty, Righty, and Slim are the nefarious three headed monster who hosts the Goosebumps Triple Header books.

Lefty
Lefty likes to create scares, or something worse. He has sharp teeth, a lizard-like face, and a Mohawk.

Righty
Righty is very cautious, he wears glasses, and has combed hair, which may indicate that he's the smart one.

Slim
Slim is the center head, possibly making him the leader. He has a completely bald head and huge sharp bottom teeth.

Trivia

 * Lefty, Righty, and Slim were the mascots of the Goosebumps Triple Header series, and the third Goosebumps mascot overall; the other mascots being Slappy the Dummy and Curly the Skeleton.
 * According to concept art, Lefty, Righty, and Slim were originally going to have pink skin, wear a red shirt, and have the look of teenage boys with more reptilian faces. It was mentioned in It Came from New Jersey! that this design was scrapped because Tim Jacobus thought the monsters didn't look scary at all.
 * Another character in the series is called Lefty: Noah Thompson, the little brother of the protagonist in Let's Get Invisible!.