(1) illustrates an optative. it expresses a wish or hope of the speaker, but there is no appeal to the addressee to make it true. (2) and (3) also express a wish of the speaker, but in both cases, there is an appeal to the addressee to help make it true, with the desired future state of affairs specified as that of someone singing. note that the person(s) supposed to sing is/are the addressee(s) in (2), thus making it a command. in (3), however, the intended singer(s) is/are the addressee(s) together with the speaker, thus effecting the modality of mutual encouragement that the speaker and the addressee(s) are to perform the action.