Wow, I didn't realize until just now just how hard this question has gotten for me. My father taught me that you only need three guns - a rifle chambered in an effective, common caliber (.308, .30-06, .270, etc.), a shotgun (12/20 gauge), and a handgun, again in a common effective caliber (as long as it was either a .357 mag or a .45 acp, my dad's words). And up until about 15 years ago, that's what I had. Three guns to fill my firearm needs; my S&W Model 19, my Savage Model 110 .30-06, and my Mossberg 500 12 gauge.
Fast forward to today, and even considering my rather small collection of bang sticks, choosing just three would be a little difficult.
For fun:
S&W 1911SC-E; easily my most satisfying and accurate handgun to 20 yds
Ruger SR556-E; able to pick off p-dogs out to 450 yds
Weatherby PA-08 20 gauge; love this gun for quail and clays
For ever:
Ruger GP-141 .357 Magnum; field gun, hunter, HD, plinker, etc.
Savage Model 110 .30-06; with the right ammo will take anything in N. America
Mossberg 500 12 gauge; might be the most used long arm I own
Not sure I could give up my 1911s or that AR though. And what about my Sig? Or my S&W Model 69? Or . . .