In the 3 shell game, it's impossible for a spectator to win, but in the 3 card monte, it's possible that they may choose the correct card? In that case, are there any outs? Thanks.

Yes, there are some, but none as convincing as with the 3 shell game. Mechanically switching the cards looks suspicious, even to a layman. If you're performing this as an exhibition, ideally, you'll never want the spectators to pick the right card. Minimise the amount of times you let them "bet" if you can, and make sure your hype is as good as you can make it.

The MacMillan Switch (or Turnover) is but one of a myriad of different options!

Of course you have to have a card held out in order to complete the switch ... but you didn't think this would be easy did you?
BTW ... done properly it not only doesn't look suspicious, it's essentially invisible.

Here's Jason England with a master tutorial in the Mac Switch:
https://www.youtube.com/watch?v=5RZ3ij2sd4o

Is there some performing 3 card monte by Garrett Thomas with clean ending?
Look forward to suggestions.