If you booked for 3 people and you show up with two people, you don't pay extra in cash to the host! You are absolutely right, @Guevara0! If they forgot to put an extra charge in for additional guests, they can't arrange for that in this way... Too bad, missed their chance!
Because it's a tough situation for when you arrive, you might want to contact Airbnb about this. Make sure all the conversation is through the messaging platform, so they can see what the host is trying to do. Flag the profile, this is even a reason you can chose in the drop-down menu!
Good luck!