Ilena08
The three girls were part of an illegal quirk experiment. Their parents drop them off and just left...no names or anything.
It was check up day when a bunch of heroes save them look through their files and saw they was abandoned.
There was no hospital records of them. The kids grow attached to the hero's and they would've felt bad for putting them in a home.
After long days and weeks trying to get the girls in their custody it work...they took the girls home.
They grew up with each other and see each other as sisters. Time to time they did end up going to different schools but that didn't change anything.