In the last eight weeks of 1981, the Federal Deposit Insurance Corp. spent twice as much money to rescue three ailing savings banks as it spent on all bank failures in the preceding 47 years.

This startling statistic was revealed yesterday at a meeting of the D.C. Bankers Association by FDIC Director Irvine H. Sprague. He estimated the assistance required from the FDIC last year for the three New York City mutual savings banks at $747 million. Seven other bank failures brought the 1981 total to $755 million, he said.

Sprague said the money spent in 1981 was $734 million more than was spent in 1980 and 2 1/2 times the $301 million losses associated with 568 bank failures since the FDIC was established during the Great Depression.

Despite these horrendous expenses, the FDIC trust fund is not only undepleted; it is in better shape than last year, Sprague said. He said that at the end of 1981, it had just over $12 billion, or $1 billion more than a year ago.

The black ink stems from increases in premiums paid by the insured banks and in interest income. In 1980 the combined total was $1.8 billion. Last year that figure amounted to $2.14 billion.

When a bank is in danger of failing, the FDIC must choose whether to close it and pay off the depositors to the limit of their insurance, currently $100,000 per account, or whether to merge it into a sound bank.

In the latter case, the FDIC usually pays the acquiring bank to the extent of the failing bank's losses. It must elect the cheaper alternative. The assistance is split so that the FDIC pays 40 percent, and the banks insured by the FDC pay 60 percent of the aid.

While this is good news for depositors who have never lost a penny, it is bad news for banks. FDIC insurance rates for banks are expected to double. In 1980 banks paid in $952 million to the fund and got an assessment credit of $521 million.

Last year they paid $1.04 billion in gross assessments, but received a credit of only $124 million. Banks paid premiums at the rate of one twenty-seventh of one percent of deposits last year. Next July the assessment will be raised to one fourteenth of one percent.

Sprague told the audience, "As banks grow larger, those that fail are larger, too. The 10 largest occurred in the past 10 years, four of them in the past 10 weeks."

The three New York City savings banks that made 1981 the most expensive year in FDIC history were Greenwich, Central and Union Dime which had combined assets of $4.8 billion before they were merged out of existence.

The new year was scarcely two weeks old when the first assisted merger was announced: $1 billion-asset Western Savings Bank of Buffalo, N.Y., was acquired by $4.5 billion-asset Buffalo Savings Bank at a cost to the FDIC of $30 million. "There may well be more," Sprague warned.

For the past two years the FDIC has advocated passage of emergency legislation to give government regulators expanded powers to deal with failing banks. Widening the number of potential merger partners would help reduce the cost to the FDIC, banks and eventually the national debt, Sprague explained.

The House passed such a bill last year but the Reagan administration and the Senate Banking Committee have held out for more comprehensive, longer range reform of banking laws.