[curtains]

In this circumstance, this save would greatly influence the strategy involved. It's a very unfair advantage to the lower stacks to make a deal like this, as one of the big advantages the giant stack has is that everytime they raise, the smaller stacks are scared to call.
Once the 2nd +3rd place guys make a private deal amongst themselves, this fear is no longer there, and your chances of stealing pots as the leader go down. I think its completely unfair and unethical, and if it's stated publicly that the 2nd+3rd place guys are trying to do such a thing, they should be forfeited, given 2nd+3rd place money, and 1st place money should be awarded to the big stack.
Regardless of whether people do it all the time, If I were to attempt to make a private deal with the other short stack in that situation, I'd consider it cheating, as I think its pretty obvious that it affects the gameplay, without the consent of one of the players.