We have a cherished pencil barreled hunting rifle that clover leafs the 1st 3 out of a 'stone cold' barrel. It won't do it any other time. It has always been that way. Who cares what the 'Real' accuracy of such a very limited purpose & little used rifle is. We know exactly what we need to know already and have no need to shoot out the crisp & shiny barrel of a 64 yr old rifle that is RIGed and put up except for special occasions and hunts.
But we have more often used rifles that knowing the the real accuracy of is helpful. For competition it would obviously be imperative.
In an article by Damon Cali, two comments of his stand out re 3-shot groups.
"A few three shot groups is just not going to get you any confidence in your rifle's accuracy." & "In no situation should you ever use 3-shot groups to get an indication of accuracy. It's just not an efficient way to spend your money or your time."
Are 5-shot groups really better than 3-shot groups?
bisonballistics.com
Of the 4 ways to measure accuracy he goes on to say real people like us only have 2 practical methods that will get us within 20% of the 'true' group size with 90% confidence, without wearing out the barrel and our bank accts.
If you have more time than money, consider using the 'Figure of Merit' Method. It uses the average of the vertical and horizontal extreme spreads to evaluate groups & is a little more efficient than the 'Extreme Spread' Method which is what we are talking about here.
For either method optimal group size is closer to 5 than 7 and for our method 5-6 groups is needed for confidence in results. For me that translates into 6 groups of 5 for regular use rifles. If I were shooting competition it would be 5 groups of a competition string, shot at the pace of my competition.
We are using 'On Target' software to assist us, thers may too.
Good luck and have fun.