Is the third shot higher or ?????
The heat factor is a very good possibility. We had a sniper rifle years ago that would literally drill em at 100 yards. But if we fired the fourth round it would be an inch off all the others sub 1 MOA group!
It seems you have good ammunition or you would not be getting the 1/2 MOA out of the first two. Also another factor is the wind. A slight gust that may not be felt at the firepoint but is possible down range. That can certainly effect the 223/5.56 at 100 yards. But as one of the guys mentioned. I would allow the gun to cool for a few minutes between shots!
AND DO NOT LOOK AT THE BULLET IMPACTS ON THE TARGET! THE AIM POINT ONLY! If you have a good rest if you think that is bothering you turn the scope power down for example. So you can more focus on your aim point and not be distracted by the bullet holes in the target. We are all human and it happens to be mentaly trying much harder for that 3rd or 5th shot since you know you have a H of a group going for you! Been there and done that!
Of course with iron sights sun on the front sight post can change things also from one shot to another.
Some pointers in Order: