Yoeman,

If you shoot a 3" 3 shot pattern 2/3 of the time, this means that almost 88% of your shots lie in the 3" circle. I would think, given the vast majority lie in 3" circle, a very minor probability (almost zero)exists that a shot will land outside a 6" circle (target).

I am assuming pristine range conditions and the sights adjusted for group centers at POA (center of circle).

That said, in adverse wind conditions, (which doesn't take much with projectiles of low ballistic efficiciency), the patterns necessarily widen (arising from wind speed fluctuation) and POI of the group centers get shifted (arising from deflection by wind). Easily with PRB, ballets, light weight PBs, this deflection can prevent the impact on a 6" target unless a windage adjustment (sights or POA) is undertaken. In other words, in adverse wind conditions, the probability of hitting a 6" target, without adjusting for wind,can go from almost 100% to almost 0%.

I don't know if this is helping, but you can develop your own probability distribution data if you keep good records of your range time and make sure the data is not dramatically affected by windage. But more important is gaining a sense of how windage is affecting POI at the more distant part of your effectiverange. Hunting in windy conditions can change your strategy, ie where your stand is, or stalking to get the wind in line with your shot.