Sean Kratz stunned prosecutors and victims' relatives last year in turning down a plea deal that would have helped him avoid the death penalty.
His cousin Cosmo DiNardo faces life in prison after pleading guilty to the killings.
Police found the missing men, ages 19 to 22, after a five-day search in 2017. Three were lit on fire and placed 12-feet deep in an oil tank converted into a pig roaster.
DiNardo allegedly lured them to his family's farm under the guise of making marijuana deals.
Kratz is charged in three of the deaths.
Jury selection is expected to last two weeks.
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