If you can be at the park as soon as they do the early opening, it's really not that bad. Here's how our day went in January. I know it's busier now, but you should be able to follow similar still:
Arrived by car at 750am
Waited at toll booth until 815
Parked, temp check, scan in by 820
Straight to MMRR with no wait until the boarding area.
Restrooms
Over to SDD. Line was just barely out to main sidewalk - waited 15-20 minutes.
Bought Food by SDD and ate
Toy story mania with no wait
AS2 with no wait
Photopass by TSL exit towards Star Wars
RotR boarding group called around this time. We had group 27. Millennium falcon wait was 60 minutes, so we skipped. Instead we explored a bit, grabbed some blue milk and sat down.
We then headed for our RotR ride. After, we went to Droid depot. I had a reservation for both myself and daughter for 1140, but they let us in at 1110. My wife helped daughter with hers.
We hung out a bit waiting for SciFi lunch.
After lunch we went to Disney Junior dance party. Great place to just have fun dancing and seeing some characters.
Then did disney visa Photopass with star Wars at launch bay.
Back to resort at 2pm for rest.
The queues in TSL are all great, your 3yo will have fun looking around and enjoying everything. Even outside the queues, the land is just a lot of fun.
If you're more excited for Slinky and are concerned higher crowds might make a longer wait if you go to MMRR first, then go to SDD first and wait til evening to catch MMRR. Also, millennium falcon waits are typically lower in the evenings as well, if you're interested in doing that ride. We did not do this one.
Overall, I think the 3 yo will have fun.