The Denver Postreported Wednesday that prosecutors announced the deal with Matthew Plake in court on Wednesday before a recess was called. Details haven't been released yet.
Another suspect, Micah Woody, pleaded guilty last week and received a 48-year prison sentence.
Two other men are charged in the alleged slayings - Josiah Sher and Christopher Wells.
Wells is the estranged husband of the woman killed and is accused of offering $15,000 to have them killed. Woody and Plake are accused of helping carry out the plot. Sher is accused of killing Amara Wells and Michael Rafferty.