Who are three heroes in The Hobbit? Could they be Bilbo (for being willing to give up his life for a better cause), Gandalf (for saving the group...

All three characters you've mentioned are certainly heroes in J.R.R. Tolkien's The Hobbit. Bilbo is a hero for saving the dwarves on multiple occasions, Gandalf is a hero for initially guiding the expedition in the beginning and for averting disaster at the Battle of the Five Armies at the end, and Bard is a hero for slaying Smaug and restoring the city of Dale. In short, your instinct regarding these characters is correct.

However,...

All three characters you've mentioned are certainly heroes in J.R.R. Tolkien's The Hobbit. Bilbo is a hero for saving the dwarves on multiple occasions, Gandalf is a hero for initially guiding the expedition in the beginning and for averting disaster at the Battle of the Five Armies at the end, and Bard is a hero for slaying Smaug and restoring the city of Dale. In short, your instinct regarding these characters is correct.

However, I'd encourage you to consider adding Thorin Oakenshield to your list of heroes. At first glance, Thorin might seem like an odd choice: he's got a reliably quick temper, tends to doubt Bilbo, and also nearly instigates a devastating battle with the elves and men of Lake-town. That said, Thorin proves his worth in the end. He cares deeply about his kinsmen, and he comes to the rescue during the Battle of the Five Armies. Indeed, this last point is key, as Thorin fights bravely and ultimately dies of his wounds. As such, though he can be an antagonistic character at some points in the story, Thorin certainly proves his heroism by the end of the novel.