Generally speaking, draw a triangle with your shots at the corners. The center of the triangle is the "zero". Once you have the triangle, draw a line from each shot to the center of the opposite side and where the lines intersect is the "zero".
You can download the free software "OnTarget" too which, IIRC, shows you where the center of your group lies.
The cause of the "flyer" in your group could be (at least):
Random Chance
You making a bad shot
Inconsistent ammo
Barrel heat
Wind
Consider for a moment that the two that are near each other each have an equally good probability of landing there randomly as did the one that's further away. 3-shot groups are notoriously unreliable but can be helpful in establishing if a load is "bad". Not nearly as good for determining if it's "good".
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Nobody plans to screw up their lives...
...they just don't plan not to.
-Andy Stanley
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