When 3 is the bbc three weekly ranking what is the airdate?
CREATE TABLE "series_6" (
  "episode_no" real,
  "airdate" text,
  "viewers" real,
  "bbc_three_weekly_ranking" text,
  "cable_rank" text
);
SELECT "airdate" FROM "series_6" WHERE "bbc_three_weekly_ranking"='3';