have you done bugs life with your 3YO? at that age they arent always able to distinguish between magic and reality. so all of the bugs and effects will be Real to a 3YO which could be extremely scary. also, some kids are much more sensitive than others. when i took dd (2.5 at the time) into philharmagic she was screaming and inconsolable within about 5 minutes. she was feeding off the other peoples reactions. she heard screaming, she sensed "great danger" or "fear" and she freaked. same reaction from watching the "
disneyland fun" segment where the kids are riding thunder mountain, matterhorn, and other rides and screaming with delight. it completely overwhelmed her because in her mind the screaming was due to fear or danger.
even if a child is not emotionally reactive like that, it can just be a very intense and *real* experience. my friend brought her almost 4yo on it and she was screaming "GET ME OUTTA HERE!!!!!!!!!!!" within 5 minutes. most of what i have read online indicates that it is not an experience designed for kids under 5-7. and even that might be too young.