That's easy. Being logicians they would take the question "do you ALL want beer" literally, not pertaining to themselves alone. So if the first two had not wanted beer, they both would have answered NO.
I agree with Chris
In Sentential Logic (smart, aren't I? ) It's a Modus Tollens argument. If P then Q; not Q; therefore not P.
Q is the idea that they don't want beer
P is them having said "No," I don't want beer."
Condition P isn't true so Condition Q can't be the corollary.
So they would have to say no. But they couldn't have known until all three of them answered and nobody objected as the third person said "YES!"
Got it right, no?!