Developing the triplets' characters can be done through their relationships. Show how they interact with Ciel, Sebastian, and the other servants. Do they have a special bond with one of the servants? Do they have different opinions about Ciel's decisions? By exploring these relationships, you can reveal more about their personalities, like their loyalty, their sense of humor, and their values. Also, consider their individual hobbies or interests, if they have any, which can further distinguish them from each other.
To develop the triplets' characters, explore their backstories. Maybe they came from a difficult past that made them so close and reliant on each other. You can also show their growth over time. For instance, if at first they were shy around their master but as they gain more experience and confidence, they become more assertive in their service. This development can be shown through their interactions with other characters and how they handle various tasks.