Interest costs of the government, as measured by weekly Treasury auctions of 13-week and 26-week bills, fell yesterday for the third time in the past month. Bills were at the lowest levels since July 6 as short-term interest rates declined on a broad front in the nation's financial markets.

Leading banks cut their prime lending rates to 19 1/2 percent from 20 percent or 20 1/2 percent. At the same time, the sensitive federal funds rate, at which banks lend excess funds overnight to other banks, declined to a range of 15 1/4 percent to 15 1/2 percent from Friday's range of 15 3/8 percent to 16 percent.

The average discount rate on 13-week Treasury bills fell to 14.198 percent from 14.412 percent a week earlier and was the lowest such rate since 14.4 percent in early July. The investment rate, or equivalent coupon-issue yield, was 14.93 percent compared with 15.16 percent for the previous bill auction a week ago.

For 26-week bills, the discount rate was down to 14.129 percent from 14.657 percent and the lowest since 14.05 percent in early July. Equivalent coupon-issue yields declined to 15.43 percent from 16.05 percent a week ago. Treasury sold $4.5 billion each of the 13-week and 26-week bills.

Discount rates understate actual investment yield because some of the price is refunded at the time of purchase, in effect.

The new six-month T-bill rate, plus a quarter of a percentage point, is 14.379 percent, the highest rate banks and thrift institutions may pay on six-month money market certificates issued in denominations of $10,000 or more beginning today. The interest ceiling for these certificates had been 14.907 percent.

The new ceiling rate for 2 1/2-year small savers' certificates, set last Monday, is 16.55 percent for savings institutions and 16.3 percent for commercial banks. A new interest rate for these certificates will be announced by the Treasury next week.