List<string> query = (from l in
                 (from t in temp
                  where t.flightdate>DateTime.Now.AddMonths(-6) && t.flightdate < DateTime.Now
                  group t by t.orgcity into g
                  select new
                  {
                      name = g.Key,
                      num = g.Count()
                  }
                 )
             orderby l.num descending
             select l.name).Take(3).ToList();