In order to drop them I'm sure you realize you would have to book a higher rate that is refundable unless you are booking a group rate. If it's a group rate, even if its refundable. What are the odds prices will be the same or lower when you want to refare the other 2 left in the cabin.
You cant just drop them and not refare the other 2. I have a couple booked where prices have doubled, what then? Someone said do a kids sail free and add a kid, idk as rarely totally free. So they could book 3 and have a no show but you want a fare back, which is iffy at best the direction prices are going. No show you get the fees and taxes back if the fake 3rd person no shows.
Check refundable rates, otherwise their deposit is gone to pay deposit for 3 and cancel 1. Sounds iffy to me.