A rifle, AR platform of some sort, if .223/5.56 get a .22LR conversion. Some sort of shotgun in 20 or 12, if a SxS look into caliber converters, but my choice would be a pump. a glock handgun or a revolver. If a glock look into caliber conversions (40 will do 9 with a barrel/magazine/extractor swap) If a revolver, something in the .357 family, and have the cylinder milled/smithed to accept moon clips for 9mm. JM2B YMMV.
just for reference My revolver choice is .327Fed, because i can use .32 s&w, .32 mag, and .327, it will even fire .32 acp in a pinch. I just look at the “only 3 guns” situation as what can I do to increase my probabilities for finding ammo.