The older BIII's shipped with a 3 x 3 combo. recently they have gone to a 4 x 3 (4blade foreward, 3 blade aft). I have never heard of mixing pitch, that would make absolutely no sense to me at all. In no calculation anywhere have I seen mixed pitch. Mixed diameter on the props, but not mixed pitch. There is no calcuation combination that is suggesting mixed pitch based on the Merc. Caclulator.
Keep in mind that pitch is the relative relationship between the distance travelled forward per a single 360 degree revolution of the prop (based on the prop rotation through a semi-solid, such as sand. It doesn't account for slip, which is introduced when rotating in a fluid.
I don't have your exact performance data, but when I run it through the calc. I come up with either a 4 x 3 22.5p or a 4 x 3 24 pitch. If you're at 22 now and hving issues, maybe the 4 blade 22.5 will help. Maybe putting in additional performance data will give you more accurate results.
Maybe everyone sitting along the transom seat along with coolers while you're tryign to get up on plane is not the best idea.
How are you distributing load when this is occurring?