It's highly unlikely that your gun is chambered for 3" loads. (You may want to read this as nearly impossible.) Should you furnish your serial number, then we can at least determine the vintage of your gun. Remember that the chamber length reflects the length of a fired round. Thus, you may well read somewhere around 3" but the old gal certainly isn't intended for a magnum shell.
As to loads... Once the actual chamber length is determined, then the ball is in your court. I can't tell you what your gun will shoot best. Some guns have rather pedestrian tastes. Others seem to like only one particular brand or load.
And that's why you need to spend some quality time at the range, not in the duck blind.