I think most of my advice would echo some that you've already received:
1) Use fastpass!
2) Make a PS for one meal per day so you know you'll be cool, full, and sitting down at least once every day.
3) Let everyone choose a few favourite things to do at each park, after you've read some good guidebooks.
4) Use the child swap if your youngest doesn't want to go on some of the "bigger" rides (One adult rides with the older kids while the other stays behind with the 3yo, and then the adults switch places. On many rides the older kids will get to ride twice, which is fun for them, too. You can do this even if you're holding a fastpass ticket for an attraction).
5) Use EE if you're staying on-site, and avoid the EE park like the plague if you're staying off-site.
6) If your 3 yo hates loud noises, consider taking ear plugs for shows or fireworks and bandaids to hide the electronic eye on the automatic flushing toilets. Without bandaids those toilets would have spoiled my 4yo DD's last vacation!
7) At least plan to the extent that you know what park you're doing on which day and which restaurant(s) you have a PS for. A plan of attack for which ride to go to in which order is helpful, too, but like Sandra says, you have to be willing to abort/re-think the mission if everyone is getting tired or cranky.
8) Bring or
rent a stroller for your youngest, even if he/she doesn't normally use one at home. You'll be able to cover a lot more territory that way.
9) Take time to hit Blizzard Beach (my favourite for a family) or Typhoon Lagoon. Some kids like them better than any other attraction at Disney.
10) Make sure everyone is wearing sun-block, hats, and drinks lots of water in order to stay happy and healthy!
Most important of all - HAVE A BLAST!