Final answer:
The simultaneous appointment of three popes in 1378 began the period in the history of the Catholic Church known as the Great Western Schism.
Step-by-step explanation:
The simultaneous appointment of three popes in 1378 began the period in the history of the Catholic Church known as the Great Western Schism. This period lasted from 1378 to 1417 and was characterized by a division within the church, with multiple popes claiming authority and causing conflicts.