I know I'm late to join this topic but if I'm reading this right, you can consistently hit near the center of a standard target at 100 to 300 yards. What's the problem with that? Unless you're shooting some match where the score is based on how tight your group is at distance all you need is two on paper, or one hit on steel.
I can routinely shoot 1/2 moa groups at 100 yrds with my rifle off a bipod but have never even given a second thought to shooting a tight group at a match. Unless you have a really evil MD setup the course, your long range targets will be 8 - 10" diameter or larger. All you need is one bullet to hit the steel, and I really don't care where it hits as long as it splats and goes "ding".
I am a relative newbie to competitive shooting and 3G and from what I've learned so far, I recommend you work more on being able to hit the long range targets when shooting from barricades, using trees / rocks for support, prone on a rock, etc... Also work on your transition from the shotgun / pistol to the rifle and getting in position / breaking that first shot as efficiently as possible.
And if you haven't shot a 3 g match yet, stop trying to get ready and just do it. Unless you train with an experienced shooter who can setup a realistic course, there is no practicing you can do that will get you ready. Just go shoot a match!