What can I say about these three? We all wish we were friends with a Harry or a Hermione or even a Ron. So what makes these three so great not only as best friends but as individuals.
Harry Potter: Obviously, Harry is our main character. The story follows him, and so the reader automatically allies themselves with Harry. He's brave and selfless and saves the world multiple times per movie. But he also has his flaws. He charges into things too quickly sometimes. He lets his emotions get in the way. He thinks he can do all these things on his own when it's been proven that things always turn out better when he's not alone. He's a tortured soul, though, because his parents were murdered and his only family left has either been murdered as well or have treated him like dirt. He's used to only relying on himself, so counting on two other people to help him tackle the big problems is hard for Harry because he doesn't want to lose more people that he loves.
Hermione Granger: Well, Harry and Ron and many other witches and wizards would have been dead way earlier in the series without Hermione. Her intellect and instinct are uncanny. Harry would have failed a million times over without her, so she's the obvious brains of this trio. Sometimes she's too smart for her own good though. Like in The Prisoner of Askaban, she uses a time-turner to take more classes and wears herself out. She's also a girl so she's very emotional just in general, and sometimes it creates problems in the friendship. And of course, she's in love with Ron, the cute bumbling red-headed idiot. It's at this point that I truly questioned her judgement, but they belong together.
Ron Weasley: Ron is the best guy friend. It's true that guys and girls can be best friends and have nothing happen between them (Harry and Hermione are a key example of that). But it's more likely to have a best friend of your same gender, and they're the ones you can really talk to about your more personal problems. I often think of Ron as the cheerleader of the group. He's brave, but sometimes the acts of bravery don't turn out well. Especially when your wand needs replacing and you try cursing Draco Malfoy. Also, he's a bit moody... okay, a lot moody. But part of Ron is also his family, who take Harry in as their own, and so he gives Harry the chance to have a semblance of a family. Harry wouldn't have survived without Ron's moral support, despite some of their fights (like almost the entire time in The Goblet of Fire).
Would Harry have survived to defeat Voldemort without Ron and Hermione? Let me know your thoughts in the comments!