I'm surprised the group didn't fall apart by the 3rd shot at that rate. What you experienced is normal. There wasn't enough time between shots to let the barrel cool. Then all you need is an ever so slight breeze to cool one side of the barrel, and it's all over the paper from that point on.
It sounds like the gun will shoot tight groups. If you wait about 2 to 3 minutes between shots, it may even do better.
Stay Safe,
Franz