It's important they are logicians, so they think very literally.
First guy could answer no if he did not want beer, as that would answer the question correctly. Since he says I don't know, it implies he does but is unsure of other two.
Etc
Discussion
what information does the third guy gain to come to a different conclusion?
The question is if they ALL want beer. A logician would take that literally and not answer for himself, like a normal person, but treat the question as one about the entire group. Not knowing a priori (one assumes) what 2 and 3 will want, he can't answer yes and be certain, even if he himself wants a beer. He must want a beer himself, or he could answer no, as at least one person will not have one. Etc
