Originally Posted by ORD_DC9

The transfer between T2 and T3, pre security, can not be done entirely inside. There is a short walk outside between the "last" door in T2 and the "first" door for T3.

It can be done all inside if one takes the T2 tunnel to the parking garage/CTA area and go back up the T3 tunnel, or use the train. I suggest neither of these options though.