Interest costs to the government, as measured by weekly Treasury auctions of 13-week and 26-week bills, fell sharply yesterday to the lowest levels since March 30 in the third decline over the past month. Reflecting a general easing of interest rates, yields on three-year notes also declined substantially.

On 13-week bills, the average discount rate fell to 12.695 percent from 13.352 percent a week earlier and was the lowest since 12.501 percent in late March. The investment rate, or equivalent coupon-issue yield, was 13.3 percent compared with 14.01 percent for the previous bill auction a week ago.

For 26-week bills, the discount rate was 12.721 percent compared with 13.619 percent a week ago, the lowest since 12.078 percent on March 30. Equivalent coupon-issue yields declined to 13.78 percent from 14.83 percent last week. Treasury sold $4.7 billion each of the 13-week and 26-week bills.

The new six-month money market certificate rate is 13.659 percent, the highest rate banks and thrift institutions may pay on six-month money market certificates issued in denominations of $10,000 or more effective immediately. The latest money market rate was computed by adding a quarter point to a four-week average of six-month T-Bill rates, a recently authorized alternative method to the usual addition of a quarter point to the latest six-month average discount. The old method would have yielded a lower rate. The previous ceiling for these certificates was 13.869 percent.

Beginning yesterday, All Savers certificates pay an interest rate of 10.77 percent, pegged to the 52-week bill rate auctioned last Thursday. The rate for "small savers" certificates, set last week, was 15.55 percent.

The Treasury Department also accepted yesterday $4.5 billion in tenders for three-year notes at an average yield of 14.43 percent and a price of $99.87. The average return was down from the record 15.96 percent on comparable securities sold in August.