First off, let me say you picked a GREAT sight! I have an FFIII on my 835, and love it.
If you are going to strip your gun down and reassemble it, I would shoot it before the season to make sure everything is lined up. Anytime you remove the barrel, you need to recheck.
Secondly, the 3 MOA vs. the 8 MOA mean miniute of angle. It means that at 100 yards a 3 MOA dot will cover 3 inches of your target, and an 8 MOA dot will cover 8 inches of the target.
Personally, I went with the 8 MOA. At the ranges we shoot turkeys, that is fine. I found the 3 dot too small for my liking, but it's really personal preference.
Good luck to you this spring!