18.104.22.168 Permission for Bination and Trination
1. A priest with diocesan faculties is permitted to celebrate a second Mass on a weekday because of the scarcity of available priests and for a just cause, such as a funeral or nuptial Mass, or to fulfill the parish schedule when another priest is not available.
2. He is also permitted to celebrate a third Mass as a Vigil Mass on the evening of the day that precedes a Sunday or other holy day of obligation  when he has already celebrated or concelebrated two other Masses due in part to a funeral, wedding or ordination Mass and another priest is not available.
3. He is also permitted to celebrate a third Mass on Sundays and Holy Days of Precept, though this should not become an ordinary practice.
 c. 1248 §1.