My 3 youngest kids were the exact ages of your 3 kids when we were at Universal last spring break. My 3yo (Jake) was 40" (barely) so could do some bigger rides, but not others. He is now 42" tall and when I bring him soon (Jake and I go next month to WDW and Universal and the whole family goes to Universal in April) he will be able to do even more. How tall is your 3yo?
In Universal the only thing we could not all do together was Men in Black. Jaws is not scary and even my 3yo who got scared in some other rides and shows liked this. I don't remember Jake being scared in Earthquake or Twister, but he was in Neutron (go figure...he thought that he was really underwater--lol). Shrek was not open yet.
In IoA he of course was not big enough for the big coasters and Dr Doom. He did do Spiderman several times. I can't say that he liked it because he felt that he was really falling, but he did ok. We did Pteranadon Flyers and he was ok on that (I was a bit scared, though, but I have a fear of heights, even though it also gives me a thrill, if that makes any sense). My 9yo LOVED PF and my older kids also liked it, but there is no express so it needs to be done very early in the day before the lines starts to build up. There is a maximum height restriction usually, but first thing in the morning they let everyone go on it in our group (my sister and BIL were with us along with my 15yo daughter).
Jake did not really care for the kiddy coasters since he's not much of a coaster kid, but my other kids would have at his age (for a 4th child Jake is really not the most daring thing, but that's ok--he's cute and I'll keep him).
My 9yo went on and liked everything that Jake did, and also liked Dr Doom, Ripsaw Falls, Jurassic Park, etc and loved the round Barge ride. The only thing he didn't do were the big coasters (Hulk and Dueliing Dragons. My older kids liked Hulk, but Dueling Dragons was their favorite coaster. We went on that so many times.
Do your older kids like big coasters? I don't know if I'll ever be able to get my now 10 yo on any coaster that goes upside down, but his older brother has gone on them as soon as he was tall enough. It just depends on the child.
My general rule is to bring them on anything that they are tall enough for once. They don't have to do it again (unless it's Spiderman

), but they can't know if they like it unless they give it a try. Having said that I still can't get my 10yo on the upside down coasters. I'll try in a year to get him on Rock N Roller Coaster at MGM.
If you have any other questions just ask.
T&B