Shimano 3 Speed Internal Hub Gear on child bike popping chain on 3rd speed my daughter got a S'Cool bike that's a 3 speed internal hub gear system. It works fine in the first two speeds but when it goes to third gear it makes a bit of noise - like a small "whizzing" sound that often comes a few seconds when changing gears before the cassette switches over. Sorry, I'm not very technical yet on my bike knowledge.

After about a minute or two of riding in third gear the chain will pop off. She's 5 years old, very average weight so there should be nothing about too much strain on the bike. I've replicated the problem by holding the bike in the air and manually activating the pedals for about 30 seconds and then chain will pop off with almost no load.

This is the model she has (though she picked a real "pride of the irish, blazing orange color) https://www.scool.de/.cms/503-1-374
Put the hub in each of it's gear selections ( esp the one where the chain falls off ).  Attempt to twist and push the cog.  If there is any play in either regard, that is probably your issue.  Could be adjustment of the shifter, but....