300 HAM'R (Converts from .223/5.56, 300BLK's Super Sonic Big Brother)
.308 WIN
30-06 SPRG
All .30cal bullets, weight ranges 95-220gr, Can always be found.. even now.
Brass for all 3 can be found on the ground at any range... even now.
Now something to consider is 3 that can use the same powder.. Not likely gonna find one, but that being said, each of the listed cartridges on their own can cover all the bases. You can almost touch 1,800ft lbs with the HAM'R. 7.62 Ride or Die Reloader
.223
.308
.300 Win mag
An abundance of components (well, there used to be) for these calibers
Factory ammo available (well, there used to be) should you at some time have to go that route
Available in a variety of platforms to serve multiple needs