What are the names of those stations?
SELECT t2.station_id FROM train AS t1 JOIN route AS t2 ON t1.id   =   t2.train_id GROUP BY t2.train_id ORDER BY count ( * )  DESC LIMIT 3
names of stations refers to selection of station_id; the operation involves joining tables train and route on matching train_id; aggregation with GROUP BY and ordering by count to find the top stations based on frequency.