20-22 teams and none had the 3rd running back get as many as 8 carries. About 5 of them didn't have a third RB get even one carry, probably 10-15 had their 3rd RB get 1-3 carries and the other couple had the 3rd RB get 5+.
You agreed, so not trying to make a point, but just thought it might be an interesting tidbit from a random sample.