What makes Harry a Gryffindor is that he ultimately chooses his loved ones over himself when he bravely walks into the Forbidden Forest to meet death face to face at the hands of Voldemort. This sacrifice is what the Order of the Phoenix, Dumbledore’s Army, and their allies an ancient magic protection against Voldemort’s curses.
What makes Hermoine a Gyffindor is she also stands up for herself and her friends but only after being back into a corner or seeing someone else is absolutely defenseless. In the final book she shows the ability to think on her feet by casting a Stinging Spell on Harry so that the Snatchers won’t recognize him and Note: If you’ve only seen the movies Hermoine was not originally written as that brave or reliable on her feet. She panicked and forgot a lot of the knowledge she had when in a threatening situation. At one point she forgot she was a witch.
What makes Neville a Gryffindor is the moment when he stands up to Voldemort while being forced to wear the flaming sorting hat. He rips the hat off of his head in a moment of direct defiance against the greatest dark wizard and pulls our the sword of Gryffindor to server the head of Nagini.
I understand why Rowling would make them all Gryffindors because if the characters were in the houses represented by their personalities then we would have missed out on a lot of the interactions in classes, the common room, and dormitories. I believe Rowling wanted to keep that aspect while still giving us a chance to dive into the depth of a student from each house.
Or I’m completely wrong… but I’m not.