Table A
   -some_id

Table B
   -another_id

Table C
   -some_id_fk
   -another_id_fk

CREATE TABLE a (a_id serial PRIMARY KEY, a text NOT NULL);
CREATE TABLE b (b_id serial PRIMARY KEY, b text NOT NULL);

CREATE TABLE ab (
  ab_id serial PRIMARY KEY
, a_id int REFERENCES a ON DELETE SET NULL
, b_id int REFERENCES b ON DELETE SET NULL
, UNIQUE (a_id, b_id)
);

CREATE OR REPLACE FUNCTION trg_ab_upbef_nulldel()
  RETURNS trigger AS
$func$
BEGIN
DELETE FROM ab WHERE ab_id = NEW.ab_id;
RETURN NULL;
END
$func$ LANGUAGE plpgsql;

CREATE TRIGGER upbef_nulldel
BEFORE UPDATE OF a_id, b_id ON ab
FOR EACH ROW
WHEN (NEW.a_id IS NULL AND
      NEW.b_id IS NULL)
EXECUTE PROCEDURE trg_ab_upbef_nulldel();