Its a giant spider with 3 heads, so I'd make an antropomorphic, headless spider and the 3 heads are both interchangeable and competing for control.
Have Baal's house/domain features a giant spiderweb inhabited by more headless spiders: the single anthropomorphic one is used as main body around, but since it bears a single stump, it can carry a single head at a time; 3-4 more of those headless spider bodies are still alive, large enough to carry a single head, they are used by two the heads who aren't in command that time of the day/month in to move around the house/lawn, while a giant headless spider with 3 head stumps lies dead and alone in the basement, the carcass looks like the dead husk of a spider, and cannot be removed as its impaled to the ground by an holy lance that no demon is able to remove, but a cluster of eggs is also present on the place, as Baal originally experimented on growing a new body for himself, but the heads slowly lost their synchrony and developed different personalities, while none of the eggs that hatched managed to give birth to a body large enough to carry all 3 at the same time, with the closest thing to success being exactly the single anthropomorphic body.
Wanna make Baal still the lord of a section of hell? Make it an unstable triunvirate, where the 3 have constant conflicts which are solved by vote or, when the 3rd head is not available to break the stalemate, in the most petty way imaginable, like a game of heads/tails, rock/paper/scissor or tic/tac/toe.