one thing to realize - it is the Italian fire code / laws that are to blame here.
Max 2 people per room - no matter how big the room is.
And the local Marriott hoteliers cannot give you permission to violate the code / law.

Others have reported the front desk looking the other way when checking in with a 3rd person (child or teen) - but I wouldn't count on it in Italy.

Italy is just not an efficient place to use Marriott points when traveling as a family.