Meinen should be used when we want to express or ask for an opinion e.g. Was meinst du? What do you think? or also : Was meinst du damit? What do you want to say with that? Kennen should be used when we want to express that we are familiar with a person or a place. e.g. Kennst du den neuen Nachbarn? (Do you know the new neighbor?) -> Ja, ich kenne ihn. (Yes, I know him) Wissen should be used when we want to express a fact, something that we have knowledge about. The answer would requiere a phrase as opposed to a noun or pronoun and most of the time you would have a subordinate clause which starts with ” dass, wie, wann, wo, wer, warum …”. Weisst du, wie alt er ist? (Do you know how old he is?)