Another big issue bedsides the dark is that it is so hard at Disney to tell real from fantasy. My cousin at this age was terrified of a lot of rides because everything seems so real. Disney is so sensory on a lot of rides that even if we say it's pretend their brain cannot grasp that.
3 year old's brain: I can touch make believe characters. Cartoons are real here. I can take a picture with Cinderella and have their autograph. The water splash during Pirates was real, so how is the cannon not really being shot towards me? I saw the ghost in my car, how is that not real. The heat from the villain's explore is hot, so that is is real.
You can do your best to explain, but their brains don't always get it. So my suggestion is doing some rides that don't blur fantasy and reality as much. My kids loved the trains, people mover, any of the up and down or spinning rides, and stay away from the dark rides. At 3 they should want to ride it's a small world 944 times in a row.