There were three of them.
There always had been, as long as anyone could remember. It had always been Harry, Hermione, and Ron. Sure, they had other friends, but those three were family before they became family by marriage.
At Hogwarts, they were known as the "Golden Trio" by nearly everyone.
Snape didn't like them because the boys reminded him too much of James Potter and Sirius Black, and the girl reminded him forcibly of Lily.
Dumbledore liked them because after all they had been through they were still normal, still together.
Remus loved teaching them because they had so much potential and because they were like the Marauders without a Peter.
Draco hated them, at first because Harry had rejected his offer of friendship, later because they had something he never would.
No matter what happened to them, through all of the rough patches they'd endured, something made them fall back on each other.
After Voldemort's defeat, life settled down for them. There were no more dangerous situations, and they liked it that way.
They all ended up at the Ministry of Magic, Harry becoming the youngest Head of the Auror Department, with Ron as his second. Everyone knew that Harry worked better when he could count on his back being covered in dangerous situations by the man that might as well have been his identical twin if it weren't for their obvious physical differences.
Harry and Ginny's children viewed Hermione and Ron as a second set of parents, and Ron and Hermione's children viewed Harry and Ginny the same way.
In later years, after all of the children were grown, the trio left the Ministry and headed back home to Hogwarts. The three of them were some of the longest serving teachers Hogwarts had ever seen.
At long last, the trials and troubles of their pasts came back to haunt them.
In their final days, whenever someone would try to separate the three, they couldn't. The connection between them, of love and of magic, was too strong.
They all ended up at King's Cross. It was where they met, and it symbolized everything to them.
Hermione was the first to go. She sat down on a wooden bench and waited. There was no way she'd leave without them.
Harry and Ron came together, just as they always had. "Ready to go, boys? The train is coming."
The boys, now looking as if they hadn't aged a day past seventeen, grinned. "We're ready. Except this time, someone else is coming with us."
A red-haired girl came running down the platform, out of breath by the time she reached them. "This is one adventure that you're not stopping me from going on." They linked arms and boarded the Hogwarts Express one last time, just as they had done all those years before.
They had been three as long as anyone could remember, and that is how it always was, and always would be in the minds of everyone. But now there weren't three--there hadn't been three for a long while anyway… now there were four.