The money supply, which had grown sharply in the first three weeks of June, declined by $800 million in the week ended June 25, the Federal Reserve Board reported today.

Yields rose for the third straight week, meanwhile, at today's offering of Treasury bills.

Today's new money figures likely will make it easier for advocates of an easier money policy to carry the day when the Federal Reserve's policymaking open market committee meets Wednesday.

With the economy in the throes of a severe recession, many analysts anticipate the Fed will take steps to lower interest rates and make available more loanable funds through the nation's banking system.

But in the last 2 1/2 months, interest rates have declined markedly and loan demand at the nation's banks has eased considerably. Some analysts think the Fed already has gone far enough in its easier policies and worry that further loosening of policy will not help the economy but will set the stage for a renewed spate of inflation.

At the same time, short-term federal government borrowing costs, as reflected in weekly Treasury bill auctions, continued to rise today. It was the third consecutive week that discount rates have increased. The discount rate for 13-week bills was 8.209 percent compared with 8.149 percent last week. r

The Fed, for its part, says nothing. But the central bank has been focusing its attention on money growth rather than interest rates since last October. The Fed could decide to make more funds available, but keep interest rates at or near their current level.

Several more major banks, including New York's Chase Manhattan and First National of Chicago, today lowered their key business lending charge, the so-called prime rate, to 11 1/2 percent.

Although the first move to the 11 1/2 percent rate took place more than a week ago, many banks have resisted lowering the rate in order to increase their profit margins. In addition, other short-term interest rates such as the interest the Treasury must pay on its bills and the interest banks charge each other for overnight loans of excess reserves (the federal funds rate) stopped their two-month decline in mid-June. As a result, bankers have become more cautious about lowering the prime rate which stood at 20 percent in mid-April.

Still, even if short-term interest rates stabilize at their current levels, the bank prime rate can be expected to come down to 10 to 10 1/2 percent within a month or so, according to Richard Everett, top domestic economist at Chase Manhattan Bank.

But Everett said the decline in short-term rates probably will continue after a brief respite. "The economy is still weakening, and very rapidly," he said.

"While I don't know what the Fed's thinking," he added, the recession should continue to push down short-term rates. Bank loans, for example, which bulged slightly in early June, resumed their decline in the week ended June 25. When demand for funds declines, so does the price (the interest rate), normally.

Last week, in a move seen as easing money policy, the Fed announced it would remove all the credit restraints it put in place to fight inflation last March, plus a special charge slapped on banks in November. Those moves make the cost of money lower to banks.

The Federal Reserve, through its buying and selling of government securities in the open market, could offset some of the easing effects of last week's moves and there is a contingent on the 12-member open market committee that thinks Federal Reserve policy has eased enough.

Bond prices fell about $20 on a face value of $1,000 today, according to Peter Goldsmith of Merrill Lynch, Pierce, Fenner & Smith.

"The markets seem to be settling down for a long hot summer," he said.