If a school says dont send more then two recommendations... how harmful is it to send a third?

It is as only as harmful as visibly demonstrating that you are incapable of following (or reading) simple instructions while simultaneously professing an interest in a career based largely on following instructions.

Touché! ;)
I still think I'll do it, though. They're all nicely stashed in one package. in the worst case they'll simply refuse to open the third envelope. In the best case the reaction will be "that bastard" followed by "wait, this is interesting! Admitted!".

Right.

I quite agree with Bender. If the school only says two letters are required, I think it is alright to send the third letter. However, if the school clearly says not to send more than two letters, why would you show them you refuse to follow their instruction?

Anyways, this is just my personal opinion so it's up to you. Good luck!