These are three words for saying the same thing. Defamation is the untruthful statement that would tend to lower one’s reputation in the esteem of one’s relevant community. Which means, somebody says something false and mean about you, and other people get the wrong idea about you because of it. That’s what defamation is.
It’s important to note that the mean, false thing, has to have been said to someone other than you. Because if somebody tells you something bad about yourself, you presumably know whether it’s true or not, and you can disregard it if it’s untrue. So, it has to be published to a third party. And it also has to be the kind of statement that could make people feel worse about you.
It’s said that Hitler cannot be defamed, because what could you possibly say about Hitler that would ruin his reputation in the eyes of history?