In reference to Macbeth, what would it mean for the play if Lady Macbeth were the third murderer in Act III?

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.