I always wonder why Medusa looked different from the other two, is there a storyline from any of the other Fate series I can be pointed to?
Eh, it's not really much of a story. I think it's discussed a bit in Ataraxia and the background information of FGO. Basically, Stheno and Euryale were born as "perfect" goddesses. They are eternal and unchanging (aside from being able to be killed). Medusa, however, was not "perfect". She started out looking like them, but eventually grew into the form we're familiar with. You can see this with her Lancer incarnation, she's a dead ringer for her sisters. If her sisters actually could age and change, they'd probably end up looking like her.
