How many seats became avaliable in Massachusetts 3rd district?
CREATE TABLE "table1_224840_4" (
  "district" text,
  "vacator" text,
  "reason_for_change" text,
  "successor" text,
  "date_successor_seated" text
);
SELECT COUNT("vacator") FROM "table1_224840_4" WHERE "district"='Massachusetts 3rd';