Hard question to really ponder, why 3? Is it the apocalypse (what type of apocalypse?) , must always be moving? Has a foreign invader taken over? What is ammo availability? With that said assuming some sort of apocalypse and ammo shortages I would choose a concealable 9 mm, a higher capacity full size 9 mm, and an AR or AK patterned rifle. My three would be Kahr PM9 or CM9, a Glock 17 (with hi capacity mags) and an AR 15 chambered in 5.56. Most of this is based on Parts and Ammo availability assuming there will be scarcities.