The Icom IC-R3 and IC-R20 can use either a custom fitted Li-Ion battery pack from Icom, or 3 standard AA batteries.  However, if you use AA batteries, there is special insert that goes into the battery compartment.   When I lost the insert for my R3, I developed this 3d-printed replacement.   It works fine in my R3 and I have received feedback from dozens of R3 and R20 users and that it works for them.   

Affordable but good, it works perfect.