You have to make the right choices on your Friday night hunting trips with Bernard to get him to share the bed with you. That gets you the first heart.
Then he will invite you on a non-date to the hot springs. Make the right choices there (repeatable twice a week) and you will get his second heart.
Then you'll be able to visit him at his home after dark (it's attached to the shop, so still choose the shop as your destination), and you'll be able to spend the night with him and have sex (repeatable), that earns you the 3rd heart you need.