Hmmmm ... I think public ridicule and humiliation are your best bet. I would suggest setting up a fake blog that describes a similar situation (make sure you date it for about two or three years ago). Seed it with a bunch of comments that don't specifically answer the red-herring question of security but answers that ridicule the question. Once you have that up and running, submit it to digg or reddit and have your friends and coworkers upvote it. Once it's on the front page, just casually mention to manager and manager^2 what you've found during your *research*. Hopefully that will flow up (or down) and manager^3 will see the question as pretty irrelevant.

Okay ... it's either that or just tell manager^3 you think his question is silly -- you want a new job anyways.

Update: Just humor people ... I think perrin's answer is probably the best so far.