An overthrow in and of itself doesn't mean it's not a hit (though any advancement of additional bases because of the overthrow are because of an error; you ain't getting credit for a double). If she's beating the throw to the base, it's a hit.
It can also possibly still be a hit even if a perfect throw would have resulted in an out. If the 3B is charging in and has to make a barehanded, off-balance play and the throw is errant, but it just barely beat the runner to the base, that's probably a base hit. If the 3B charges and makes the tough play and throws it away when the runner was only halfway to first? Probably an error.
It's subjective, and it's impossible to give an opinion without seeing the play.
It can also possibly still be a hit even if a perfect throw would have resulted in an out. If the 3B is charging in and has to make a barehanded, off-balance play and the throw is errant, but it just barely beat the runner to the base, that's probably a base hit. If the 3B charges and makes the tough play and throws it away when the runner was only halfway to first? Probably an error.
It's subjective, and it's impossible to give an opinion without seeing the play.