We go to Disney every year, so here is my advice for you!
Clothes:
-Anything lightweight and cool. Disney in July is going to be unbearably hot, and you want the stuff that isn't going to cling. Sundresses, tank tops, etc. As for shoes...sneakers are great, but they will make your feet hotter. If you have sandals with straps on them, they are best for all of you. Lands End has cute ones for kids that are around $20 and worth every penny. My son walks all over Disney in them and has never complained that his feet hurt.
A stroller is 100% necessary for a 3 year old. It will be hot, there will be lots of walking, and he will be exhausted. You do NOT want to have to carry him around as you will be just as hot and tired. I suggest taking a lightweight stroller with a good umbrella shade if you have one. A large stroller is a pain.
You can bring any/all food you want in to the park. If you are doing the meal plan, you probably won't need to bring much of anything in as you get plenty of food throughout the day. If you aren't, then I suggest things that don't melt easily. Sandwich crackers, granola, etc. are all good choices. Also, any place that has access to a faucet will give you free water. There really is no reason to ever have to buy it, so we always just take the free stuff! :)
If you have any other questions, please feel free to PM me! I have helped tons of moms on this site, and LOVE talking Disney! Have fun! :)