The Third Buddhist council was convened in about 250 BCE at Asokarama in Pataliputra, supposedly under the patronage of Emperor Ashoka. This is however disputed, as mention of the council never appears in the Edicts of Ashoka.
The traditional reason for convening the Third Buddhist Council is reported to have been to rid the Sangha of corruption and bogus monks who held heretical views. It was presided over by the elder monk Moggaliputta-Tissa and one thousand monks participated in the Council. The council is recognized and known to both the Theravada and Mahayana schools, though its importance is central only to the Theravada.
The account of the background to the Third Council is as follows: Emperor Asoka was crowned in the two hundred and eighteenth year after the Buddha's parinibbāna. At first he paid only token homage to the Dhamma and the Sangha and also supported members of other religious sects as his father had done before him. However, all this changed when he met the pious novice-monk Nigrodha who preached him Appamada-vagga verses from the Dhammapada. Thereafter he ceased supporting other religious groups and his interest in and devotion to the Dhamma deepened. He used his enormous wealth to build, it is said, eighty-four thousand pagodas and viharas and to lavishly support the bhikkhus (monks) with the four requisites. His son Mahinda and his daughter Sanghamitta were ordained and admitted to the Sangha.
Eventually, his generosity was to cause serious problems within the Sangha. In time the order was infiltrated by many unworthy men, holding heretical views and who were attracted to the order because of the Emperor's generous support and costly offerings of food, clothing, shelter and medicine. Large numbers of faithless, greedy men espousing wrong views tried to join the order but were deemed unfit for ordination.
Despite this they seized the chance to exploit the Emperor's generosity for their own ends and donned robes and joined the order without having been ordained properly. Consequently, respect for the Sangha diminished. When this came to light some of the genuine monks refused to hold the prescribed purification or Uposatha ceremony in the company of the corrupt, heretical monks.