I'm sure you can get better than a 3" group at 75 yards from a Marlin lever action. I did some tuning on one in .35 Remington, and it would shoot into 1" @ 100 yards with the Speer 180-grain flatnose. I took a little metal off the receiver end of the magazine tube (to keep the expanding barrel from changing the pressure exerted by the tube onto the receiver face as the barrel heated up), free-floated the barrel as much as I could, and reduced the trigger pull. I think the same or a similar treatment would improve the grouping of yours as well-if it indeed needs any improvement! Shoot it first-you may be surprised!!

I had a Win. M94 in .44 Mag., and used my revolver loads in it - 22 grains of Hercules 2400 and the 250-grain Keith 429424 semiwadcutter. I had to reduce the length of the cartridge-stop projection on the cartridge elevator so it would accept theis load-the overall length is longer than factory ammo. But after this change, the action functioned well. This load was really hell on armadillos!

(BTW, I'm aware of a Ruger M96 lever-action in .44 Mag., but have never heard of a Marlin M96... only the 1894.)