when was the premiere when a 3.3 millions of North American watched the episode whose writer was Rob Wright?
create table table_21313498_1 (original_air_date varchar, us_viewers__millions_ varchar, written_by varchar, PRIMARY KEY (original_air_date))
select original_air_date from table_21313498_1 where us_viewers__millions_ = "3.3" and written_by = "rob wright"