Year of Three Popes

The Year of Three Popes is a common reference to a year when the College of Cardinals of the Catholic Church are required to elect two new popes within the same calendar year. Such a year generally occurs when a newly elected pope dies or resigns very early into his papacy. This results in the Catholic Church being led by three different popes during the same calendar year.

The most recent instance of a Year of Three Popes occurred in 1978. The three popes involved were:

There have been several instances in which three or more popes have held office in a given calendar year. Years in which the Roman Catholic Church was led by three different popes include:

There was also a year in which the Roman Catholic Church was led by four popes, called the Year of Four Popes:

...
Wikipedia