Chan is used in very friendly relations or very young girl. Kun is used relations between men and men or your boss calls you when you are re sponsoble to you! San is use in very friendly

San: To everyone, polite and respecting the opponent. School, company, community, used everywhere. Kun: To male who is in a similar age with you, or to male who is younger than you. Used at school, or between friends mainly. Chan: To very close female friends. Sometimes to male friends as a part of nickname e.g. Daisuke (male) --> Dai-chan. If a male boss calls a lady with chan as in Yuka-chan, it sometimes can be a sexual harrassment because she doen't want to be called in such a way by him. She thinks in her mind, "I'm definitely not your friend!!" I hope my explanation will help you some :-)