Question: How many trains call at Castor, Overton, Peterborough East and are operated by LNWR?
Schema: create table table_12221135_3 (arrival varchar, operator varchar, calling_at varchar, PRIMARY KEY (arrival))
SQL: select count(arrival) from table_12221135_3 where operator = "lnwr" and calling_at = "castor, overton, peterborough east"