PRETORIA, South Africa--Four Black activists, three of them leaders of the country's largest anti-apartheid group, were convicted of treason yesterday. They could face the death penalty.

The verdicts culminated a three-year trial which focused on the role of the United Democratic Front, a nationwide coalition formed in 1983 to press for Black political rights. The state alleged that the front incited anti-government violence in Black townships in 1984. The coalition's leaders said they advocated peaceful protest.

Three of those convicted of treason were senior front leaders who had spent 40 months in jail without bail--Popo Molefe, 36, who was the coalition's national secretary; Terror Lekota, 40, who was its chief spokesman, and Moses Chikane, 40, a leader of its Transvaal Province branch.

Also convicted of treason was the Rev. Thomas Manthatha, an activist in an area called the Vaal Triangle south of Johannesburg where the first wave of unrest erupted in September 1984.

Sentences will be issued after mitigation arguments scheduled to begin Dec. 5.