[V]

4ch rears have holes in the brake caliper backing plates where the abs sensors(2) mount in on each side of the rear. Those sensors pick up readings using a cog style ring pressed on the axles.

3ch rears have a hole in the top of the differential housing, above the gears, where a single abs sensor mounts and that picks up its readings off a ring on the carrier.

Also, you should be able to use 3ch axles with 3ch brake backing plates in a 4ch housing. That setup would result in no signs of a place to mount an abs sensor. You would have to look for parts numbers then and ensure its a 4th gen rear by measuring the width (4" wider than a 3rd gen rear).