The front ends set which 2 bands the scanner could receive and they are printed circuit boards that plugged into the mother board. The scanner has mounting spots for 2 of the 3 available front ends. The crystals plug into the front end boards. The 3 front ends that were available covered Lo-band, Hi-band and UHF so you could customize the scanner to cover any 2 or the 3 bands.
As I mentioned before, your BC-III uses a 10.8 MHz IF (Intermediate Frequency). Most of it's contemporary scanners used a 10.7 MHz IF. I don't know why Bearcat was different unless they wanted to lock you in to buying their crystals.