If you're planning on coming as a 3, there are a few different ways of booking this. You can book one 2 person cabin and a solo spot. The person in the solo spot place would be sharing a bunk bed cabin with someone else who has also booked solo. If you make the booking on the same account, you will automatically be assigned to the same yacht.
Alternatively, you are welcome to book two double cabins however the full price for 4 people would have to be paid so it may work out a little expensive.
If neither of these options suit you, you could find 3 available spots by looking at the Crew Finder where already existing bookings reach out for other crew members to join them. The Crew Finder Group on Facebook is also a good place to look for spare spaces on existing yachts.