Welcome to disABILITIES
If you haven't looked at the
disABILITIES FAQs thread, I'd suggest looking there. You will find a couple of links in that thread that should be helpful. One is about hints for dealing with ADHD - I realize that autism is not the same, but some of the same hints that are helpful for people dealing with ADHD will be helpful to your family. There is also a link to a trip report called "Adventures in Autism" on that thread and there is a section about GACs (Guest Assistance Cards).
There are also 2 books that would be helpful. One is
WDW For Kids By Kids, which is exactly what it says - WDW from a kids' view. I know a 3 yr old is not going to read it, but that book has reviews of rides from kids and includes things like a 'scare factor' that might help you to pick appropriate attractions for your child. The other book is
Passporter's Walt Disney World for you Special Needs (that is being updated and a new version with a new name will be coming out soon). That book covers all sorts of special needs, so it might look a littlem overwhelming, but it does have a lot of helpful information.
In most cases, you won't have to worry about characters coming up to you. Most of the characters are either in a specific place/building where you need to get in line and go in for 'meet and greet' or they are set up outside with lines you can join if you want to meet them. I don't know if your child watches any TV shows, but there is a Disney Sing-Along called
Disneyland Fun that includes views of some of the attractions, music and children interacting with the characters ( it is a bit old and is
DisneyLAND, rather than WDW, but that won't matter much for what it shows).
For what would be the best attractions to go on, it might be helpful to know a little more about your child - does he enjoy music? Does he enjoy motion? In general, is a he a sensory seeker or a sensory avoider?