Your 3yo can ride (and will enjoy!) almost every ride. There are only a few he may be too short for. There are height restrictions for a few rides. If he's 40" tall he'll be able to even more.
Some he may especially enjoy:
Buzz Lightyear's Space Ranger Spin
Dumbo
Magic Carpets of Alladin
Many Adventures of Winnie the Pooh
Goofy's Barnstormer
Even your infant can ride most of the rides. If there's a ride that she can't go on, the two adults can take turns riding. You can do a "baby swap" where one of you rides while the other one waits, and then the second one rides without standing in line again. Ask the CM (Cast Member - employee) at each ride how to do it. Every ride is a little different. At Splash Mountain the second adult waits in a little play area, and then goes through the Fast Pass line (which is usually much shorter than the regular line.) If your 3yo is tall enough to ride Splash Mountain (40"), he'll be able to go twice in a row.