Who were the opponents in the match that was played on a hard court and had a runner-up outcome?
CREATE TABLE "doubles_6_3_titles_3_runners_up" (
  "outcome" text,
  "date" text,
  "surface" text,
  "partner" text,
  "opponents" text,
  "score" text
);
SELECT "opponents" FROM "doubles_6_3_titles_3_runners_up" WHERE "surface"='hard' AND "outcome"='runner-up';