The mayday came minutes after firefighters arrived at a 3-alarm blaze that’s believed to have started about 9 p.m. Thursday with an overloaded extension cord in the bedroom of a second-floor apartment at 11 Ashton Place in Methuen. One of them was trapped — and running out of air. A rapid intervention team trained specifically to rescue firefighters was en route from Salem, N.H. But time was running out for Lt. Thomas Curtis, a 57-year-old father of four who has been a firefighter for 29 years. Luckily, Lt. Michael Hughes was nearby.