If your talking Encore I would pick 3 that I do not own. They would be
1 .223
2 30-o6
3. .375 or 20 gauge
I tend to pick cartridges that are less common place than these for my own use, .222, 250 Sav, .338-06, but if I could only have 3 these would allow me to handload up and down for anything. I have an old Savage 219 with several barrels, Hornet, 30-30, 410, 20 and 12 gauge and it comes pretty close to being an all around gun. It is interesting that this old idea is coming around again. Shotgunner