A Treasury statement named the three as retired Israeli military official Israel Ziv and South Sudanese businessman Obac William Olawo, for leading entities whose actions have extended the conflict, and South Sudanese official Gregory Vasili, "for actions that have undermined peace, stability, and security."
The US also designated six entities owned or controlled by two of those named, without giving details.
There was no immediate response from South Sudan's government, which has bristled at rising US criticism and pressure, including the threat to withdraw aid.
The statement said the US is targeting people who have "provided soldiers, armored vehicles, and weapons used to fuel the conflict." Washington has expressed growing exasperation over South Sudan and those it says are blocking the path to peace. It led efforts in the United Nations Security Council to impose an arms embargo on the country earlier this year.