"What have we got today?" Harry asked Ron as he poured sugar on his porridge.
"Double Potions with the Slytherins," said Ron. "Snape's Head of Slytherin House. They say he always favors them –– we'll be able to see if it's true."
–– Harry Potter and the Sorcerer's Stone
Double seems to mean two Houses together. When three houses together, do you use triple class?