I haven't seen anybody mention this, but if your older kids like riding thrill rides, then I think the key is to make sure the 3-year-old is at least 40 inches.
At
Disneyland, the following rides have a 40" requirement: Space Mountain, Splash Mountain, Big Thunder, Star Tours, Radiator Springs, Soarin', Tower of Terror, Jumping Jellyfish, Symphony Swings.
Those are some of the most fun, iconic rides at any amusement park anywhere. Our 3.5 year old loved everything he could ride, even Tower of Terror. Of course, every kid is different.
Rider swap can help, and it means the older kids can get double rides sometimes, but it is much worse at Disneyland than at Disney World, because of tighter timing constraints. It also means separating the family, and it takes way longer for everybody to ride everything. And it can create stress when you're constantly trying to split up and meet back up, or when the riding party knows that the waiting party has been waiting longer than expected. It's more fun for the family to be able to stay and ride together.