What is the average number of runner-up that National University, which has more than 2 total championships, has?
CREATE TABLE "number_of_championships_by_school" (
  "university" text,
  "men_s" real,
  "women_s" real,
  "runner_up" real,
  "total_championships" real
);
SELECT AVG("runner_up") FROM "number_of_championships_by_school" WHERE "university"='national university' AND "total_championships">2;