Ummm, no. I tend to be a stickler for following the rules. Plus, you will not be able to take advantage of any extra magic hours as everyone has to show a room key in order to get in, even the little ones. So you loose that perk of staying on-site. And you will not be able to sign up for the dining plan, if that interests you at all. And God forbid there is a fire, and Disney thinks they only need to look for 2 kids, not 3.
Wait, I'm confused. What resort are you staying at? Only the deluxe resorts have queen beds, and most of them allow 3 kids in a room.
Really, if it is a problem, you should follow the rules. If every family snuck in an extra body there would be a big strain on the services. Busses would be packed, the pools would be packed, the restaurants would be nuts. It is not ok for you to not follow the rules, because then it would be ok for everyone to not follow the rules. And then their would be a big mess!