Question: Who were the writers of the episodes directed by Greg Sullivan and which first aired on July 21, 2012?
Schema: create table table_29087004_3 (written_by varchar, directed_by varchar, united_states_original_airdate varchar, PRIMARY KEY (written_by))
SQL: select written_by from table_29087004_3 where directed_by = "greg sullivan" and united_states_original_airdate = "july 21, 2012"