What was the distance of the race in which Northerly raced in group G3?
CREATE TABLE "2002_03_season_as_a_six_year_old" (
  "result" text,
  "date" text,
  "race" text,
  "venue" text,
  "group" text,
  "distance" text,
  "weight_kg" real,
  "jockey" text,
  "winner_2nd" text
);
SELECT "distance" FROM "2002_03_season_as_a_six_year_old" WHERE "group"='G3';