Funny - I was just thinking about this this morning. My list was going to be five, though, so I'm having to whittle a bit.
1. Marlin 336 in .30-30. It's plenty powerful for either hunting or home defense, it's relatively light (compared to a Garand!) and fast handling. While not semi-auto fast, the lever action is fast enough for follow-up shots. Ammo is plentiful and, best of all, I reload for it and have both cast and jacketed bullets on hand. It's a natural pointer for me and as "just a lever-action" it doesn't set off people's Black Gun radar.
2. A .22 revolver. My Smith model 14 would be my choice right now, because that's what I have, but if I were shopping I might go with a stainless Ruger Super Single Six with convertable .22 Mag cylinder. Revolvers don't have mags to lose and while slower to reload (especially the single-action), they have all of the accuracy of an auto.
3. This one is a toss-up. I like the idea of a more powerful handgun for a carry gun and in that case, my Ruger SR9c would probably get he nod. It's relatively light and thin and very accurate. Plus, with the extended mag, it'll hold 17 rounds of +P 9mm shoudl the need arise. If the Ruger wasn't available, my Mossberg 500 in 20 gauge would be the alternate.
It is tough if you think about it and honestly consider the options. Back in the day, Skeeter Skelton asked that question, but only ONE gun. His pal, Dobe Grant, answered with a .22 rifle after whittling the list down from a 12 gauge, a .30-06, a .44 Magnum, and a .22 revolver.
I'm grateful I live in a place where I am not limited to three, five, or even ten so I don't have to choose and limit myself.
Q
