I can totally relate to being apprehensive about transportation. We went to WDW with our 4 and 6 year olds. We stayed at Wilderness Lodge and Contemporary to be as close as possible to the Magic Kingdom, and I still left there a little bitter about the whole experience. It cost a FORTUNE, and the only really great memories I have of the trip are Hoop De Doo and the character dining.
At three, there is plenty to do at UOR, and it is so much more relaxing and easier on the parents, physically, mentally, and budget-wise. We went to UOR 13 months later when they were a year older and it was SO much more fun and easier than WDW and SO much more budget friendly. We became annual pass holders and 5 years later have no plans to return to WDW, especially since word is it's now so much WORSE than 2019.
I know I'm in the minority here, but the play areas at UOR are so great for little kids. Yes, there's more rides for them at WDW, but that entails lines, planning, criss-crossing the parks and/or added expense. The playgrounds are there for them instantly, with no lines. My kids still LOVE the spray guns at Camp Jurassic and spend at least an hour playing with them each trip. We have to drag them away.
If you want a lot of rides for littles AND easy transportation, my advise would be to research
Disneyland. Our experience at DL was far superior to WDW except for the character dining. It is so easy to get around DL and the other visitors are so much more chill than at WDW. Everyone is so stressed out at WDW, trying to have the BEST DAY EVER, lol. The locals give DL a completely different vibe. And Cars Land at DL is AWESOME if you have a little Lightening McQueen fan.
Good luck with whatever decision you make. You know what's best for your family. I just wanted to let you know you're not crazy for wanting to make this trip as easy on yourself and your kid as possible. You will be knocking yourself out for this kid for decades to come. It's okay to sacrifice a few dark rides he won't remember to have a balanced trip everyone enjoys.