WASHINGTON (AP) — Average U.S. rates on fixed mortgages declined this week for a third straight week. The low rates could give a boost to the spring home-buying season, which has gotten off to a slow start.Mortgage buyer Freddie Mac said Thursday that the average rate for a 30-year loan eased to 4.20 percent from 4.21 percent last week. The average for the 15-year mortgage fell to 3.29 percent from 3.32 percent.Mortgage rates have risen nearly a full percentage point since hitting record lows about a year ago.Warmer weather has yet to boost home-buying as it normally does.