Rifle: Something chambered in 7.62x39 because it is so widely available, and I can carry more of an intermediate cartridge than I can of a larger rifle cartridge. From my current collection, that would mean one of my SKS carbines. Probably the Chinese one, since it's lighter and easier to handle than the Yugoslavian one.
Handgun: A quality 9mm full size pistol, because 9mm ammo is so widely available. A .45ACP would be my second choice, but I don't own any .45ACP pistols yet. Out of my collection, I'd probably choose to take my HS2000 (same gun as Springfield XD9), because it's a full size gun with 16rd magazines. I prefer my Bersa Thunder UC9 for daily carry, but if it's a battle/shtf scenario, I'll give up the extra concealability of the Bersa for the higher capacity and longer barrel of the HS2000/XD9.
Shotgun: I only own an H&R single shot .410, but if I were going to buy a shotgun for combat/shtf, I would go with a reliable pump 12 gauge like the Mossberg 500/590 or a Remington 870.