When DS was a baby and right up until 4 years the shows/ ride he liked were Animagique, the parades, the carousel- the carriage, not on one of the horses!, It's a Small world- we went on that many times in a row, plus cafe Mickey for brilliant character interaction.
I can't remember the others off hand.
PLayhouse Disney is good. DS loved that when he saw it in Florida (haven't seen it at DLP yet).
It's been a few years since I went to DLP, so I can't remember what else was suitable for littlies, so hopefully someone will come along later who has more recent experience.
With 3 kids, I would advise bringing sling, and then you'd be hands free to help with getting your other two little ones on and off rides.
Disney is magical for little ones, and in my opinion, it's the atmosphere, characters and music etc that make the trip, DS can't remember the rides from visits when he was tiny, (except iasw) but he remembers meeting Mickey and having a sword fight with Captain Hook.
In answer to your 1st question. If you plan your holiday well and don't expect to go on every ride and are happy to take it easy, then it won't be a hard holiday. We found Disney to be the perfect baby holiday. So much easier than a UK holiday, as your kids will be entertained all day and happy children = happy mum and dad! PLus the baby change facilities etc are fab, and in restaurants they will bring out the kids food first, so they don't have to wait.
HTH