WASHINGTON » Long-term U.S. mortgage rates fell this week for a third straight week, posting new lows for the year. The benchmark 30-year rate reached a three-year low.

The low rates come amid the spring home buying season, luring prospective purchasers.

Mortgage buyer Freddie Mac said today the average 30-year fixed-rate mortgage dipped to 3.57 percent from 3.61 percent last week. It’s far below its level a year ago of 3.85 percent.

The average rate on 15-year fixed-rate mortgages eased to 2.81 percent from 2.86 percent last week.

Prices of U.S. government bonds have been at high levels since the Federal Reserve’s recent decision not to increase the benchmark interest rate — which it had raised from record lows in December. Some economists believe the Fed may not raise rates again until the second half of the year.

That means low levels for the bonds’ yields, which move in the opposite direction from their prices and tend to influence mortgage rates.

The yield on the 10-year Treasury bond stood at 1.73 percent Wednesday, down from 1.77 percent a week earlier. The yield rose to 1.75 percent today morning.

To calculate average mortgage rates, Freddie Mac surveys lenders across the country at the beginning of each week. The average doesn’t include extra fees, known as points, which most borrowers must pay to get the lowest rates. One point equals 1 percent of the loan amount.

The average fee for a 30-year mortgage was 0.5 point, down from 0.6 point last week. The fee for a 15-year loan was unchanged at 0.5 point.

Rates on adjustable five-year mortgages averaged 2.78 percent this week, down from 2.80 percent last week. The fee was steady at 0.5 point.