1) What for? Practically every edition of Zhukov's memoir has some differences from the others. Since it was the "main" memoir, some of its content was changed from time to time to reflect current political realities. It's kind of like the official "History of the Second World War".
2) The last edition has the biggest amount of changes -- at least an entire new chapter was discovered in manuscript form among Zhukov's papers. For more on that, see Karpov's biography of Zhukov.
3) This is the last edition:
http://militera.lib.ru/memo/russian/zhukov1/index.html (2002) It's the one I have, and it's supposed to be the most complete one.