Probably up to the day prior. Pending availability, of course. Availability is determined by
DCL as 1) does the stateroom have bedding space for 1 more, 2) does the muster station have space for 1 more, and 3) if <18 is the appropriate kids' club maxed out. Since this is a 3rd passenger, #1 is no problem. With the lower capacity #2 may be a question. And age.