3 hours... thats it.... you probably won't have time for the shows- not sure what time the first one is...The Clyde & Seamore show is great fun for kids, but would probably eat up a lot of your time... obviously, Shamu would be great, but, same problem.
In terms of what not to miss:
See the dolphins, (go into the underground area so you can see them from below, too), be sure to check out the stingray pool (near the dolphins), feed the sea lions, DON'T miss the penguins; not sure what else you'll have time for.
SeaWorld needs a whole day!!!