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.
The other extreme is another red herring, often put forth by trolls to confuse the issue. No one needs to have natural born citizen parents in order to be natural born citizens themselves. Those parents must be citizens, period, and naturalized before the time of the child's birth will suffice.
Minor v. Happersett states that children born of the soil of citizen parents are without a doubt natural born, and anyone else is in doubt. Any other definition is speculation without precedent, or deliberate twisting of legal decisions such as claiming that Wong Kim Ark was determined to be a natural born citizen, when he clearly was not. He was deemed a citizen, period, with no additional qualifiers.