Not enough information to answer the question accurately?
There is not much real world difference between the .308 and the 30-06 - except the ought six shoots heavier loads better than the .308. The .308 is reputted to be more inherently accurate the the 30-06, however individual rifles will dispell that myth. The .300 Win Mag is in essence a souped up 30-06, again adding a little speed and trajectory over the 30-06 beyond the 300 yard mark.
What are you going to be shooting that you want to zero the rifle at 300 yards. If you take any of the three calibers you are contemplating and zero them between 1.5 and 2.5 inches high at 100 yards, you should be very close to dead on at 200 yards, and somewhere in the neighborhood of 6.5 to 9 inches low at 300. If you zero at 300 yards, you will be shooting over the back of most big game animals at 100 and 200 yards. 90% plus of whitetails are still shot under 200 yards.
My choice for any of the three would be a quality 3X9X40 one inch tube fully multi coated scope.
For the money, I would put a Bushnell Elite 4200 3X9X40 with rainguard on the rifle and never look back. Other choices might include the Zeiss Conquest, the Weaver Grand Slam series, or the Burris Signature Select or even the Fullfiled II.
My 2 cents worth.