Hi Crocuta, 性格 is the general word for personality. 人格 also means personality, but this is used more for moral topics. 性質 is normally used for things, and even animals. Even though dictionaries say you can use it for people, I've never heard people say, for example, "彼女はおとなしい性質です", especially in a daily conversation. I guess academic people may use 性質 for people. Hope this helps!