In my home poker games
we are all friends. One guy always had HORRIBLE luck. This was a friendly game where buyin was $5, and only a few reloads are ever needed each night (usually by this unlucky guy)
He is a good player, but would keep running into trips with two pair, full hous vs flush, etc..
So he went all in once, and I had the nuts, I folded. It was just money between him and I, he could have re-bought for like $20 if he wanted to, so me giving him the $9 pot didnt hurt anyone else. (edit I have a chart somewhere, I was up like 150 on our games, while everyone else was below or just above even)
I dont consider that collusion because we never had a secret agreement
and it definitely wasnt for an illegal purpose... as far as being deceitful... didnt seem that bad
Edit: Oh and if you tell the third person in on an all in to check it down, that is not very secret.... if you talked with him before the game and said, anytime someone is all in, check it down, then that would be collusion in my opinion.
As far as I've noticed, unless there is a lot of money in a side pot, or someone hits the nuts, most people check down, though Ive knocked two people out at once before when the side pot had enough to warrant some action and some guy tried to steal it instead of checking down.