Three mathematicians walk into a bar.

Three mathematicians walk into a bar. You'd think the second one would have ducked.

This joke is an alternative "walk into a bar" joke - the type wherein the bar being walked into is a long hard object. The joke both is a pun on the word bar, and also an insult against mathematicians - the assumption being that mathematicians are oblivious, and would therefore not notice the other mathematicians walking into the bar.