Being limited to three, I'm thinking.....
#1 Shot Gun
Reliable and rugged, can be used to kill anything from very small to very large with the proper loads. Think food or aggressors.
#2 Assault Type Weapon
Reliable and rugged, large capacity for fending off multiple zombies.
Reliable and rugged, high capacity 40 cal.
Optional #4 Long Range Rifle
Scoped Bolt action 30.06 common round, good killer and ability to fight the boogies at a distance.
I know you said three...but I'd sell the girlfriend to make room for the 30.06.