SCHEMA:
CREATE TABLE table_21313498_1 (original_air_date VARCHAR, us_viewers__millions_ VARCHAR, written_by VARCHAR)
USER:
when was the premiere when a 3.3 millions of North American watched the episode whose writer was Rob Wright?

QUERY:
SELECT original_air_date FROM table_21313498_1 WHERE us_viewers__millions_ = "3.3" AND written_by = "Rob Wright"