Just out of curiosity, is there a topic where even an excellent graduate student wouldn't be able to master all the necessary background within 3 years (I say 3 because in many European countries a PhD is meant to last three years beyond the 5-year masters, with perhaps a fourth year to finish writing-up) ?
For example, I have heard that mastering EGA or Lurie's books takes several years, but does that mean some topics are simply out of reach in those European countries, and require an american-style PhD that lasts 5 years or more ?