I am also of the opinion that you can never have too many, but if I had to get by on the absolute bare minimum this would be my choice.

17 HMR (plinking/varmit)
25-06 (longer range varmit/thin skinned game)
30-06 (anything else on the face of North America)

I love my 270 but think that the 25-06 in this case fills the void more efficiently. An article was published in Rifle Shooter I believe about this.