RE: Hot Barrel?
A good rule of thumb to determine if your barrel is "hot" is if you can't hold a finger on the barrel near the chamber for more than 2 seconds. I highly doubt that your barrel was getting hot enough to effect your groups.
I don't have the answer to why your 3rd shot keeps "throwing", but I'd bet a dollar that barrel temp. isn't the culprit! I'm curious as to how far off the lands are you seating your bullets. Might try to seat the bullets either closer to the lands or further away if you're already seating right at the lands! Just an idea.