Wrong, FO3 has no relation to either of the former Dogmeats. Dogmeat died at Mariposa, canon. The FO2 Dogmeat was an easter egg found in a non-canon special encounter.
I think someone at Bethesda said that he was supposed to be a descendant.
That's when I actively ignored that and decided the Lone Wanderer was like "Oh hey, there's this dog here! Hmmm... I have some dog meat with me... I know, I'll call him Dogmeat!". Because that infinitely makes more sense than it being a descendant.
Oh by the way, I chose Rex. I would prefer a regular dog with a different breed that isn't named Dog Meat for Fallout 4 though, we've had 3 cyberdog and *sigh*... 3 Dog Meats.