What you need:

For Beavis:
Metallica t-shirt (choose one)

For Butt-head:
AC/DC Back in Black t-shirt by Junk Food
(or other AC/DC shirt)

For Stewart: Winger t-shirt

Optional props:

Nachos

Candy or cappuccino (for Beavis as Cornholio)

"Burger World" visors or name badges (you can make these with a few craft supplies)

Be sure to continually use these lines while in costume:
"Huh huh uh huh", "Fire! Fire!", "Shut up, Beavis", etc.