In all honesty I would book both a room for 2 and a room for 3 each with a refundable deposit.
If you book a double and want to add a third you will need to book a different category and most likely pay a lot more.
If you book a triple and cancel one guest then most likely they will make you book a double occupancy room at the current rates.
You can cancel and get a full refund up until 90 days prior to sailing.,