What is the RAvg when the ratt is 3?
CREATE TABLE "post_season" (
  "year" text,
  "team" text,
  "comp" text,
  "long" text,
  "rate" text,
  "r_att" text,
  "r_yds" text,
  "r_avg" text,
  "lost" text
);
SELECT "r_avg" FROM "post_season" WHERE "r_att"='3';