The weakening economy set back sales of existing homes for the third consecutive quarter from July through September, says a real estate trade group that expects little improvement for perhaps six more months.

"The market will remain slow for the rest of 1990 and into the first half of 1991, as the country experiences a mild recession," said Norman A. Flynn, president of the National Association of Realtors.The Realtors said sales of previously owned homes dipped 2.5 percent to a seasonally adjusted annual rate of 3.66 million units in the third quarter compared with the same period of 1989.

Existing-home sales also fell in the first two quarters of 1990.