Деп айту usually follows after any word or sentence and means "to say smth. like this". For ex., Барамын деп айту - To say that I will go. Деу means to say or to say smth. For ex., Сау бол деу - To say good bye. Деген depends on the context it's used. If we use it according to people, for ex., Ануар деген адам, this will mean The man who's name is Anuar. Secondly, if we use it to places, for ex., Алмалы деген ауыл, then in this context it will mean The village which is called or named Almaly. We see that in previous cases it is used in the meaning of "who is" or "which is" and "what is". And thirdly, деген is also might come as a verb (past time of деу used for the third person). For ex., Ол парктемiн деген - She said that she was in the park. Деу - to say. Деген - said.