United Airlines rejected a call by federal mediators that it resume talks with its striking pilots, dashing hopes for a quick settlement in a walkout that has crippled the nation’s largest air carrier for nearly three weeks. United said in Chicago that it had informed the National Mediation Board that it would not attend a proposed meeting today with mediators and the Air Line Pilots Assn. in Washington. The board’s call was a request, not an order. Representatives of the 5,000 striking pilots had agreed to the meeting.