WASHINGTON — Interest rates on short-term Treasury bills were mixed in Monday’s auction: Rates on three-month bills dropped to the lowest level in three weeks, while rates on six-month bills were unchanged.

The Treasury Department auctioned $32 billion in three-month bills at a discount rate of 0.100 percent, down from 0.105 percent last week. Another $28 billion in six-month bills was auctioned at a discount rate of 0.150 percent, unchanged from last week. The three-month rate was the lowest since these bills averaged 0.085 percent on Oct. 1.