Are priests permitted to concelebrate the Triduum at the EF Mass?
In the Extraordinary Form, any priests present during the Triduum besides the Sacred Ministers (priest celebrant, deacon, and subdeacon) simply attend Mass in choir. They may also assist at the chanting of the Passion on Good Friday by acting as deacons.
All who chant the Passion must be at least deacons. The fullest form of chanting the Passion involves three deacons, (or priests vested as deacons) besides the Sacred Ministers, chanting the Gospel. If three deacons are unavailable, two deacons and the priest celebrant may also chant the Passion, with the celebrant chanting the part of Christ from the Gospel corner of the altar. The celebrant and one additional deacon may also chant the Gospel, with the deacon chanting both the Synogaga and Chronista parts. The priest celebrant alone may also chant all three parts. The priest celebrant may also simply recite the Passion Gospel, even at a sung Mass.
The choir, whether a formal liturgical choir or a lay choir in the loft, may sing the parts of the Turba, with the clerical Chronista singing the parts of individuals (e.g. Peter, Pilate).
On Palm Sunday and Good Friday, during the singing of the Passion Gospel, are laymen permitted to sing the role of the Synogaga or Chronista?