Almost every ride, 3 can fit in one row - esp 2 young kids and an adult. Even the ones that can't have 2 or more rows anyway so 5 yr old can sit right behind you or in front of you.
As far as what to go on, don't know how tall 3 yr old is or how brave. My just turned 3 dd wanted to go on EVERYTHING but was 1/4 inch shy of 40" so she couldn't do Splash Mountain or Thunder Mountain. I would never let her do Space Mountain or Tower of Terror even if she was 40". I think they are too scary for that age. She did do Kali Rapids in Animal Kingdom which has a 38" requirement. She LOVED that.
Magic Kingdom (stay mostly to the back and right of the park)
-Fantasy Land must rides:
-carousel
-dumbo (you can all fit)
-winnie the pooh (can all fit or be in same 2 row honey pot)
-peter pan flight (you can all fit)
-snow white (can all fit but 3 yr old DD was SCARED - it is scary)
-small world will be closed
-Toontown
-barnstormer rollercoaster. That might be tough to fit 3 though
-the rest is a ton of fun, no rides per say but kids will love it
-Tomorrowland
-Speedway (have 5yr drive while you push pedal w/ 3yr on lap
-buzz lightyear - you can all fit in a row
-astro orbiter (3 of you should fit - not sure if 2 is max though)
-Lilo and Stictch may be open by then
**other stuff to try there is Goofy's Country Jamboree show, Jungle Cruise, Aladdin's carpet ride (like Dumbo), Country Bear Jamboree, Swiss Family Treehouse, Haunted Mansion, Pirates of Carribean, Bell's storytime, Spectromagic parade, Wishes fireworks at night.
Animal Kingdom (dd's 2nd fav park)
-Safari was great, dd loved that
-Kali Rapids if everyone is above 38" and likes to get wet!
-Lion King Show
-Tarzan Show
-Train ride
-Dinoland is a blast for kids - there is a playground area, rides, carnival games etc.. that is a don't miss.
-Pocahontas and her animal Friends (we didn't get to see this)
MGM
-Playhouse Disney
-Beauty and the Beast Show
-Muppet's 3-D
-Ariel - Little Mermaid Show (dd was a little frightened by this)
-Another playfround called Honey I shrunk the Kids Movie Set, my dd would have stayed there all day - both will love it!
-lots of character meet and greets!!!
-Fantasmic is loud and scary for some young kids. If you plan on doing this, do the Fantasmic package so you don't have to wait in line to get a seat.
Epcot
Living Seas
Honey I Shrunk the Kids 3-D
Imagination with Figment (can ride together) and the Imagination Station has lots of cool stuff
Mexico ride (dd loved this one - a lot like small world!)
Illuminations is LOUD and scared a lot of small kids. Be prepared.
I think overall for a 3 & 5 yr old, you would spend most of your time at MK, AK, MGM then Epcot. Remember to take breaks in the middle of the day, plan sit down meals since waiting in line then searching for a table IS a nightmare!
HTH