Prosecutors planned to seek the death penalty for Sweat, but settled for a jury trial waiver in exchange for his life.
Sweat's fate will now be determined by an Okfuskee County judge.
Sweat stands accused of murdering Skyla Whitaker, 11, and Taylor Paschal-Placker, 13, and his fiancée Ashley Taylor.
Whitaker and Paschal-Placker were found gunned down by the side of a county road in 2008.
Ashley Taylor's charred remains were found after she disappeared in 2011. According to an arrest affidavit, Sweat confessed to cutting her throat.