Since I haven't read 'Black Hammer 3 Graphic Novel', I can only guess. If it follows the pattern of the previous installments, characters like Talky - Walky might be main characters. Also, the heroes who have unique powers and personalities, like Madame Dragonfly, could be among the main cast. They might be facing new challenges or having new interactions that drive the story forward.
I'm not sure which characters are the main ones in this specific volume. But in the Black Hammer series in general, there are likely to be characters like Abraham Slam, Golden Gail, and Colonel Weird. They could very well be the main focus in this graphic novel as well.
Without reading the 'Black Hammer 3 Graphic Novel', it's difficult to determine the exact main characters. But in the Black Hammer universe, there are a variety of interesting characters. Some of the more prominent ones could be Barbalien. He's an alien in a human world, and his story arc could be continued or expanded in this graphic novel. Another possibility is that some of the lesser - known characters from earlier in the series could be given more prominence and become main characters in this volume, as often happens in a series to add new dimensions to the story.