I do, as does a majority of native born white southerners. It's even almost true for native born black southerners; that 3G grandparent is likely to be a 14th Amendment citizen, with successive generations being natural born.
Parents don't need to be natural born, however, and you certainly should know that by now, you've been told repeatedly for going on two years now. Parents just need to be citizens, by whatever means, when their child is born in the United States for that child to qualify without a doubt as a natural born citizen.
If that 3G grandparent is a 14th Amendment citizen, then according to the ludicrous definition put forth by jarofants, none of their descendants would EVER be natural born citizens even until the ending of the world.
Moreover, anyone of the “pure” American bloodline would have to hunt high and low to find a mate of that “pure” American bloodline; or none of their children, grandchildren, great-grandchildren, etc; until the end of the world, would ever be natural born citizens.