I waited until DS was 6, which I think was a perfect age. DS could ride almost everything, and the few things he was too short for probably would have made me motionsick! Although he definitely knew the characters were actors, it didn't make a bit of difference in his eagerness to meet them, and collect their autographs.
I'm not sure either DS or I would have enjoyed WDW when he was 3. He would have wanted to be up, touching things, playing, not sitting in his stroller watching the crowds go by, or standing in line. At that age he could have had so much more fun in 4 hours at the park or the pool or the beach than he would have had waiting in line over and over again.
If I did take a 3 year old it would be only to M.K. and probably only on a EMH morning so that we could do the fantasyland attractions without long lines. I could see DS at that age enjoying a couple of hours of fantasyland, plus some time to play on Donald's Boat, followed by a really early lunch at the Crystal Palace, and leave by about 12:30. BUT that's a really expensive morning! (a couple of hundred to stay on site, plus adult and child one day tickets, plus the character meal). For the same price I could buy him season tickets to a children's museum, or take him to the beach for a whole weekend -- both of which he'd enjoy WAY more.
To me the #1 thing on vacation is spending time with my child, doing something we both enjoy. Sounds like either the plan you posted (hanging out at the resort without going to the park, lots of pool time and a few character meals) or a few days at the beach would fit that bill perfectly!