Not every third degree felony is identical to every other third degree felony. There are ones where the circumstances of the case are going to warrant a longer sentence, and those are probably going to overlap with the ones where the circumstances of the case would warrant charging a juvenile as an adult.
You are assuming that what leads to a child being charged as an adult is the curcumstances of the crime, and that they will have a longer sentence for those lesser charges instead of different charges that reflect the sentence.