Since Lady Macbeth is not identified as the third murderer in Shakespeare's Macbeth, and since the only way she could be the third murderer is to be portrayed as such by a production of the play, and since Lady Macbeth being the third murderer makes no logical sense, it would mean that some director and actors took some silly liberties in interpreting the play.
Or, if you're asking what it would have done for the play had Shakespeare written in Lady Macbeth as the third murderer, the answer to that is that it would show Shakespeare isn't as good as we think he is.
You see, Lady Macbeth can't even kill the king, because the sleeping Duncan reminds her of her father. If she can't kill Duncan, she certainly isn't going to attempt to kill Fleance. Plus, by this time in the play Lady Macbeth has been shut out of the decision-making process. She doesn't even appear to know about the murders until after they occur.
Finally, Lady Macbeth makes plans that work. She is the planner of the family. Duncan's assassination comes off with no problems, until Macbeth varies the plan and kills the grooms. The plan to kill Banquo and Fleance fails, and the plan was ill advised in the first place. Lady Macbeth would not have devised this plan.