I'd go to PP then go to ETWB. You'll probably have a short wait but PP should be a walk-on and it only takes about three minutes to do the ride.
ETWB is a bit of a "storytime" but not like at the library. You go first into Maurice's workshop, where you watch a short clip and say a little thing ("take me back to the day the beast and Belle fell in love," or something.). The effect to transition from the workshop to the next room is one of my favorite effects anywhere in all of WDW... And I say that as a true Disnerd adult.
Next room is with the wardrobe, who assigns parts to the kids (and anyone else who wants to participate.). The whole group gallops like Phillipe, growls like the Beast, etc. They make sure any kid who wants a part gets one (but no one has to have one.). "Parts" are designated by holding up picture cutouts of whoever they are - so like a 1' square cutout of Chip, etc. Grown ups can have a part too if they so wish, lol.
Then you go into the library, where you sit on benches and Belle comes out. The story is "told" with the characters coming forward - and then everyone parades around the room. I don't think a 3yo would have any problem sitting for this part, either on a bench or on your lap. The kids get an opportunity for a quick (very quick) photo op with Belle and then it's out the door.
Yes, 15-20 minutes is probably accurate, but because of how it's broken up, I don't think any kid would have an attention span issue.
Hope that helps!