50 years ago it was 1970. At that time the Australian Open was not relevant so players competed for 3 slams a year. Slams were not the be all of tennis Other events were relevant but the vast majority of tennis fans don't know about them nor care about them, and judge these players on the...
Nadal had it tough with Djokovic who blossomed later though. He only became dominant 6 years after Nadal first slam. He certainly was lucky in recent years with the weakness of all newer generations.
Sampras high percentage is also a factor of him failing to reach more slam finals.
They competed against other players, it has nothing to do with the big 3. They competed with Agassi, Sampras, Rafter, Kafelnikov, etc. Murray has yet to defeat Pete Sampras in his first slam final, age 20, which both Safin and Hewitt did.
Better in what sense? Total career achievement? In that case, group B is better, despite most of them having very short careers and despite Murray being in group A. Is it better in term of peak level and the ability to give trouble to the best players? In that case, group B is better again.