I really like the article that was posted.
IMO, it's kind of like deciding which side of the car is safer if you're outboard. In some instances, the driver side is safer. In some instances, the passenger side is safer. I am sure that in come collisions, it's better to be in the 2nd row, whereas in other collisions, it's better to be in the 3rd row. Since we can't choose our collision, we do what we can and hope for the best.
And even if something came out that said "the 3rd row is ___% less safe than the second" I'm not sure I could/would do anything about that. There are a LOT of things I do every day that I could do more safely, but practicality wins and I have to just do what I can. (We could stay home on days it rains so that I don't have to drive on wet roads, or we could skip going to the park so that my son wouldn't fall off the playscape, but that's not really reasonable, and avoiding sitting my children in the 3rd row isn't reasonable either.)