What category is Matthew Barton?
CREATE TABLE "singles_6_3_titles_3_runner_ups" (
  "outcome" text,
  "date" text,
  "category" text,
  "surface" text,
  "opponent" text,
  "score" text
);
SELECT "category" FROM "singles_6_3_titles_3_runner_ups" WHERE "opponent"='matthew barton';