IMO, Shooting 3" lead loads out of a 12ga. is unnecessary for upland hunting and certainly out of a 12ga. that is 6lbs or under. I could see a case for a 20 ga. loading in 3" which happens to be 1 1/4 oz IF your trying to emulate a 12 ga load out of a 20, but if you need 1 1/4 oz, why not just shoot the 12?
Steel shot may be another matter do to larger shot and pattern density.