Answer:
Black had been the potter's secret-keeper
Step-by-step explanation:
Three broomsticks is a very popular inn near Hogwarts, the school Harry Potter studies. He and his friends frequent this region a lot. One day Harry spied a conversation with his teachers on the three broomsticks and heard a news that made him very sad and worried. He heard that Sirius Black had been the potter's secret guardian.
This happens in the third book in the series titled "Harry Potter and the Askabam Prisoner"