The Rites of Holy Week by Fr Frederick McManus, published in 1956 by the St Anthony Guild Press, Paterson, NJ, states that, the "Passion is divided into 3 parts: the narration (C for Chronista), the words of Christ (+), and the words of the crowd or of anyone else (S for Synogoga)...The choir (even of lay persons) may take the part of the crowd; the 3rd deacon then sings only the words of individuals (i.e. Pilate, Peter, etc)." (page 69).
McManus states that "the restored order of Holy Week does not provide for the case in which 3 deacons are not available to chant the Passion. It would seem that then the deacon of the Mass should read or chant the Passion in a clear voice, since the entire Passion now constitutes the Gospel of the Mass."
In the Ritus Simplex the priest singing the Liturgy (i.e. Missa Cantata), has the option of just reading the Passion Gospel on Palm Sunday and on Good Friday instead of singing it. If he chooses to sing it, he must sing all three roles, but if a choir is available they could sing the parts of the crowd (Synagoga) that are not the voices of individuals (i.e. Peter, etc)