Actually having to live and work amongst grumbly bears and coastal brown bears on a fairly regular basis, the 12 gauge is the hands down winner of the three mentioned. A 492 grain, .730" diameter piece of lead moving at about 1600 fps makes a really big hole going in and keeps on doing so all the way. Any expansion just sweetens the deal. You'll also be shooting a pump or autoloader from the shoulder a lot faster and with more accuracy than a lever or bolt. Not pretty, not the classic rifleman's arm, but does the job.
You can buy .45-70 ammunition that is right over the top of course - but you can also buy specialty 12 gauge ammunition like the Dixie Xterminator stuff that runs along the lines of the old Brit Paradox loads, except even more so.
If you're talking about 50 yards, 100 yards, etc, you aren't talking about bear defense, you're talking about hunting. In which case a 30/06 is just fine when combined with common sense, as thousands of dead grizzlies can attest to. Perhaps not the best choice, but perfectly adequate. I've seen friends drop grizzlies quite decisively with .270's, as far as that goes. But that was back in the early 70's, prior to magnum-itis, the Internet, and bear defense threads, when Jack O'Conner was doing that and we didn't realize how crazy we were to attempt such an outrageous feat.
I never did shoot a grizzly with a 30/06, and now I feel much better hunting grumbly bears with a 358 Norma Magnum - which does a very nice job indeed. But I carry a 12 gauge, not the 358, when working in grumbly bear country.