I am a plat and have a megabonus certificate - cat 1-5 expiring end of March 2018. I have a couple of hotel that fit the category level. However, they don't have rooms for 3 people ( 2 adults and 1 child) available on category 1-5 certificate. Is it possible to book a capacity 2 room and then upgrade using points to capacity 3 room?
If what you mean is pay for a room in cash and then use points to upgrade to a different room configuration then it may be possible but I cant remember if that is a valid use case for CAT 1-5 properties with limited room configurations. A quick call to Marriott would confirm this or not.
Good luck! If all else fails, I hear folks on this site having some luck in extending their certificates to allow for additional flexibility in bookings.