All 2 3/4 shells.
Voila. All 3 have different heights, shortest is about 3mm shorter than the tallest.
Had this issue many times, certain high brass shells are slightly taller and won't fit as many as birdshot. Even a millimeter or 3 will make the extra one not fit when you put in slighter larger shells. 7 x 3 mm is 21mm and the last one won't fit.
