That's quite an age range. I went with an almost three year old and 9 month old once and what they could do was quite different. The rides in common were Dumbo, Carousel, It's a Small World, the railroad and monorail, Casey Jr. Train, Storybook Canal, Winnie the Pooh, Little Mermaid, some of the things in Bug's Life Land, and the character meet and greets. There were other rides that they could do (e.g., PotC, HM), but the 3 year old wasn't ready for them. As YukonMommy said, you can do any ride that doesn't have a height restriction as a family, so it's really a matter of choosing the rides that you and your family will like.
We also watched the parades and the fireworks (the noise didn't bother either child at the time, but some kids are sensitive). We also watched WoC, but the 3 year old had trouble seeing and got bored. The shows (Aladdin (although the snake might scare the 3 year old - the one I went with got scared), Mickey and Magical Map, and Playhouse Disney) might also be fun as a family.
It's also worth mentioning, take advantage of the Baby Centers in each park. There are changing tables, a place to nurse, high chairs, etc. It was a nice sea of calm in the parks and it was great. They also have a training potty (not auto-flush and lower to the ground), if you're working on that with the 3 year old.