Orcs, Elves, and Men
Orcs always lie, elves always tell the truth, and humans can do anything they want. A human, an orc, and an elf are hiding behind a wall.
One voice behind the wall says: "Tee hee! 1+1 = 3!"
A different voice responds: "You're wrong, human. Stop acting like an orc."
And, finally, a third voice pipes in, "Hmm... actually, 1+1 = 2."
If each of the 3 creatures spoke exactly once, who said that 1+1=3?