@Mark207
You broke your own rules . And it's a bit late now but you can try the tips above and hope they accept or you will have to call ABB (I hope you have the no., otherwise google it or search on this platform - it's only about the third of all asked questions 😉
Next time don't even let them into the room and say it doesn't work that way, we will have to change the reservation, but one of you is going to have to find another accommodation.
Let's assume your insurance only covered 2 persons (always a perfect reason), then you would now already be in violation if something happened.
Before they even move into the room, take your device, computer out and make a change to 2 ppl. in their presence. Have guest accept right then and there with no excuses, and then have a suggestion ready for a motel or whatever nearby for the third person.
And only then do the 2 guests get to check into the room.
You need to see yourself as a tiny business in room rental and handle things that way - friendly, never break your own rules, act professionally.
Your baker wouldn't give you 3 loaves of bread just because you only paid one?