No experience with this, but I assume both of your other boys are still using training wheels? or atleast one of them is?
Even if they're not I guess...
Stand there with your youngest and watch them ride their bikes. IMO your youngest will watch and want to feel apart of the group and will pick it up.
In sum: Kids are very visual, and will do anything to fit in with their peers.
Make it a group activity, have your other two help teach the youngest if you want...
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