He deduces both 1 and 2 must want a beer. Fact he can then answer yes means he also wants a beer and can confidently assert they must ALL want beer, the waiter's question.
Perhaps this varient will help clarify:
nostr:note1k7cd0f8d52lrjxshwh3fauqtpdlw5wsxdxcwmlv55kz4rqlv9vhqs4g2cu
But what information does he use to deduce that 1 and 2 must have a beer?
If all 3 enter into the joke in the "not-knowing" state [ 0 0 0 ]
then through the dialogue, information must be imparted to make it [ 0 0 1 ]
but they say "I don't know"
which afaict doesn't give 3 any information to make a deduction.
(not to over-intellectualize your joke or anything)
