I have to agree with a hunting rifle a sub MOA three shot group gives me the warm fuzzies. However I loose those warm fuzzies if it doesn't keep the same level of accuracy the next time I'm at the range. I shoot a lot of three shot groups and get a high level of accuracy with most of my hunting rifles, if I'm doing my part on the trigger.
The real test will be the next few targets you shoot. I'd go to the range bang out another three shot group with your old target stapled behind the new one. If the next three shot group is sub MOA and overlaps the old three shot group then I'd start feeling better about my rifle and my ability to shoot it. If you do that for one or two more range sessions after that and you keep all the rounds under an inch with the same POI you have a sub MOA rifle, but I'd be pretty happy if it didn't go over 1.5" for 9-12 rounds.
That will only work however if you haven't adjusted your sights and cleaned your rifle. If you adjusted your scope than you have to start all over again. If you cleaned your rifle I'd at least shoot a couple of fouling shots before I started shooting for groups.
The problem is a lot of the time I'm able to consistantly put togheter a sub MOA three shot group, and the next time I'm at the range I'm unable to get the same load to group as well. I blame it on me most of the time and most of the time I'm correct something has failed in my mechanics. I have to agree one three shot group is nice but doesn't make a sub MOA rifle, 3-4 three shot groups same POI and under an 1" does make a sub MOA rifle.