Pglasgow -
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).
What I was describing was a sequence of 3 shot groups,
not within a specific 3" circle, but within 3 inches of each other. The individual 3" circles, on successive outings, may be in varying places on the target. Can you find a probablility of 6/6 on a specific 6" circle from this information? I'm not sure.