SQL Selección de datos de tres tablas relacionadas

Deberá JOIN las tablas, así:

SELECT products.description, orders.quantity, orders.price
FROM detail_orders
  INNER JOIN products ON (products.prod_id = detail_orders.prod_id)
  INNER JOIN orders ON (orders.id = detail_orders.or_id)

Si desea consultar por un usuario específico:

    SELECT products.description, orders.quantity, orders.price
    FROM detail_orders
      INNER JOIN products ON (products.prod_id = detail_orders.prod_id)
      INNER JOIN orders ON (orders.id = detail_orders.or_id)
    WHERE orders.iduser = someUser