A 3" group is fine as long as you can do it in the field also. There are some people who can shoot a 1" group in the back yard but have a tough time hitting a "8 inch vital area on a live deer. Consistancy under pressure is the key. If you can take one arrow every day and hit your deer target in the vitals every time at 20 yards you're almost good to go.
You'll never know how you will shoot on a live deer until you put yourself
in that place. Keep practicing to tighten those groups to the best of your ability but at the same time learn to stay calm when that big buck is standing 20 yards from your stand.