what is the state for reason for change is resigned may 15, 1820?
CREATE TABLE "table1_225099_3" (
  "state_class" text,
  "vacator" text,
  "reason_for_change" text,
  "successor" text,
  "date_of_successors_formal_installation" text
);
SELECT "state_class" FROM "table1_225099_3" WHERE "reason_for_change"='Resigned May 15, 1820';