how does torn hero interact with stun abilities? Torn hero says that if an effect an opponent controls would commit a foundation, they commit that card instead. The "instead" part makes it confusing since stun does not let the attacking player choose which foundations to commit.

You can choose to commit torn hero from your opponents stun ability, then their ability has caused torn hero to be commited, which triggers its continuous ability. So there is no need to discard momentum.

I think what he's asking is, if the opponent does "stun 3" can hit discard 1 momentum, and commit a torn hero instead of committing three foundations.

No, Torn Hero will only replace one foundation being committed. In other words, don't use the response against Stun... just commit it

but you can protect 1 from going down. For some reason if you only have 2 left, and tehy stun 3, cant you protect that one last foundation from going down? Just like Akuma, how if they commit 4 to commit 4 of yours but you only have 3 up, you can protect one from ever going down with torn hero's ability