Americans had to pay an average of 15.47 percent interest for home loans on new single-family houses last month, the third straight month above 15 percent, the Federal Home Loan Bank Board reported last week.

The average rate had dropped from 15.38 percent in early January to 15.34 percent in early February before climbing again last month, the sixth increase in the past seven months, the board said.

The 15.47 percent commitment rate was almost 3 percentage points higher than the August 1980 low of 12.52 percent and 3/4 of a point above March 1980.

The board's survey of major mortgage lenders showed that the average rate on actual loan transactions for a single-family home was a new high of 14.24 percent, up from 13.94 percent the month before. The previous high rate on single-family mortgage loans was 14.01 percent in May 1980.

Behind the new increase, the board said, were continuing troubles in the savings and loan industry, which is the source of a majority of home loans.

"The recent rate rise appears to have reflected weakness in deposit flow to thrift institutions and increases in the cost of savings to these major mortgage lenders," the bank board's report said.

Those problems were underscored in a letter to members of Congress by the U.S. League of Savings Associations.The letter, signed by Executive Vice President William B. O'Connell, urged Congress to give the Federal Reserve Board regulatory authority over money market mutual funds, which the S&Ls believe are taking away business through offers of higher returns.

Assets of the unregulated funds have grown about $33.8 billion since Jan. 1, while federally insured S&Ls "experienced an outflow of $2.4 billion," O'Connell wrote.

"Policymakers must balance the desire by fund managers to maximize yield to attract customers against our nation's need for a stable financial system and credit availability across America," he wrote.

Spokesmen for the money market funds have suggested that regulations limiting S&L interest rates should be eased rather than new regulations being imposed on the funds, a move O'Connell said would not help the industry.

"Savings associations hold 90 percent of their mortgages at rates of less than 12 percent," he wrote. "Increased money costs would simply worsen the earnings crisis now facing savings and loan associations."

The new average mortgage rate for early March is another blow to the slumping housing industry. Earlier government reports showed housing starts, new house sales and building permits for future construction all down in February. Housing industry spokesmen say little improvement can be expected before the final quarter of this year -- and then only if most interest rates decline in the meantime.

The bank board's average mortgage rate is computed by surveying large lending institutions offering 25-year mortgages covering 75 percent of the purchase price.