[SoothingDave]

They can’t know if they “all” want a beer until all three have made their decision. So the third one can say “yes.”

[Arthur McGowan]

Not quite.

When each of the first two says “I don’t know,” he is telling the third one that he wants a beer. Because if either one of the first two had not wanted a beer, he would have answered the question, “No.”

[OneWingedShark]

Re: #3
They can’t know if they “all” want a beer until all three have made their decision. So the third one can say “yes.”

That's only half of it — if either of the first two didn't want a beer then they could have answered the question ('No').