Wouldn't that have to be the third one? If two walk into the bar, the second one can duck. But, it'd be too late. He already walked into the bar.

 

lol... I suppose it's how you interpret... i had originally heard it as 2 men walk into a bar (first part making you think a beverage type bar) then the job is that it is a metal bar... but logically the third ducks would probably make more sense lol

 

But again, what if they walk into an actual drinking bar, and the second one ducks, just for no reason, or to avoid a thrown fist- the first joke would make perfect sense