Cerberus wasn't the lady, it was the lady's pet.
I'm pretty sure that the pet was only the minions (the ones that skeletonized people). During the first flashback, they make sure to point out that the heroes are the spearman, the ice sword lady (this one), and the
three "magical swordsmen", which to me strongly implies that the swordsmen got fused together (hence, the three-headed cerberus) because of the order they're mentioned to the reader. The beastification would also make sense considering that the one we see, at least, is an animal-eared person.
Ofc, she does call the pet "cer-chan", but with how I think it's presented here, I think it's more likely that she (+the other two) took a form based on the pet. Otherwise, we have three heroes left, who are stated to be weaker than the ice sword lady, which doesn't seem like how arcs would typically go order-wise.