St. Andrews and two young men survived with minor injuries.
In court, St. Andrews admitted he had been drinking beer and smoking marijuana before the crash. A blood-alcohol test after the crash found his blood-alcohol content to be 0.12 percent. The state's threshold for intoxication is 0.08 percent.
He admitted he stole the car he crashed from his uncle's home.
Washington County District Attorney Kevin Kortright said he opted to drop the murder counts in light of concerns over whether his office could prove St. Andrews acted with "depraved indifference toward human life" before the crash.
Kortright also said he didn't want the victims' families to have to go through a trial.