She does have 3.

2 in her hand, one in her foot.

But the answer is: make the character more unique

Well, the real answer is so she can look cool but different.

In universe, she was cloned partially, and the DNA was altered along the way, based on a damage sample (iirc, been a minute since I read that stuff).

The third claws are in her feet. No idea how/why they thought that made sense, but if you look at Daken’s weird claws, it’s pretty obvious that some of the comic creators are batshit.