tutti i 3 modi sono corretti

Actually, it seems that they are all correct. They almost mean the same thing and they can be used interchangeably, but: if you use the second or the third one I could think that it was your choice to miss the meeting; in the second sentence "mancare" refers to "appuntamento" whereas in the third sentence, since you've used the preposition "a"(at), what you've missed is supposed to be the place of the meeting For example: 1) purtroppo ho perso l'appuntamento..; 2) ho volutamente mancato l'appuntamento; 3) sono volutamente mancato all'appuntamento; However the differences are very subtle! I hope I've been helpful! Ciao!