Since no one has I'll ask it - Where, how, and in what terrain do you plan to hunt?
All three will work well, but each may have an edge in specific circumstances:
.280 - Open Country
.308 - Steep Country or Extra Hiking
.350 - Grizzly Country or Heavy Timber
If you take a backup consider that the .308 and .350 are near enough trajectory twins.
Truth be told, I'd wimp-out and take your much lighter Montana, almost every time; but you might be tougher than me.