We'll name the logicians A, B, and C to simplify.
If A wants a drink, A doesn't yet know whether B and C want a drink. Thus A cannot answer "yes".
If A does not want a drink, A would say "no" because at least one of A, B, and C don't want a drink, making "does everybody want a drink" false.
By saying "I don't know" A indicates their personal desire for a drink, but his lack of knowledge about the other two.
B is essentially in the same situation. B can deduce that A wants a drink, but still doesn't know the preference of C, therefore must answer "no" if B doesn't want a drink, and must answer "I don't know" if B wants a drink.
C, however, can now deduce that A and B both want drinks.
So if the third logician wants a drink, they say:
Yes, everybody wants a drink.
If the third logician doesn't want a drink, they say:
No, not everyone wants a drink.