To create unique characters in 3headed dragon fanfiction, consider their backstories. A character could be from a family that has a long - standing feud with the 3headed dragon. Another could be an outcast who befriends the dragon because they both feel alone. Also, think about their appearance. A character could have a special mark on their body that is related to the dragon. And don't forget about their goals. One character might want to steal the dragon's eggs for power, while another wants to protect the dragon at all costs. Their diverse motives will make them unique.
You can give each head of the dragon a distinct personality. For example, one head could be brave and impulsive, another could be wise and cautious, and the third could be mischievous. For other characters, make them have unique skills related to dealing with the dragon. Maybe a character is a dragon - whisperer who can communicate with the dragon telepathically.
For creating unique characters in 3headed dragon fanfiction, start with the dragon itself. You could make it have a unique color pattern on its scales for each head, like one head has golden scales, another has silver, and the third has black. Then for human characters, think about their cultures. If the story is set in a medieval - like world, a character from a nomadic tribe might approach the dragon differently than a knight from a castle. Also, give characters unique abilities that are not common. For instance, a character who can see the future in the dragon's eyes when it looks at them. This will add depth and uniqueness to your characters.