The realistic answer, without nostalgia, is a .22LR rifle, a 12-gauge shotgun, and a Glock 19.
The .22 is the ubiquitous small game harvesting and training tool. The vast majority of Americans live on a budget and other, bigger rifles are dead weights. In contrast, a M1903 or a M1917 or a Garand or a M1A/M14 doesn't give the average gun owner much value. Those guns are around $700-1000 at the low end and go up to whatever you please on the high end. You can't use a .30-06 or a .308 for home defense unless you own vast amounts of surrounding land and you're defending yourself outside of the home - try shooting a .30-06 or a .308 indoors at night. Besides, bolt actions are obsolete except in very narrow circumstances like big game hunting or precision marksmanship. Most people don't have the time or money to become precision marksmen or CMP grand masters or big game hunters. What about an AR or AK? That would be a fourth gun and this list is restricted to three. But almost everyone can afford .22 rifles and ammo.
I don't think there's much debate about the 12-gauge but if you think a pistol grip Mossberg is a viable tool, please put down your crack pipe and go attempt actually using such a gun.
The Glock 19 will get the job done at a decent price for almost all possible jobs where a pistol is necessary. Concealed carry? Check. Open carry? Check. Woods gun? Check. Competition? Check. Teaching newbs to shoot after introducing them to a .22? Check. Home defense? Check.
That being said, Americans have the freedom to buy what they want and do what they want, so we shouldn't be telling people what they should own. We're 50 states for a reason - Americans like choice and freedom.