In my personal opinion, I think the third murder could be Lady Macbeth. It is said and expressed throughout the play that Macbeth and Lady Macbeth are equals and share, and tell everything with each other. But when macbeth does not tell her about his plan to kill Banquo and his son Fleance I suspect she might have been a little angry and decided to be the third murder and let Fleance escape.
I have some more pieces of evidence to support my first statement. When Macbeth and Banquo received their prophecy they were the only people who knew about it, but Macbeth wrote a letter to Lady Macbeth informing her of the prophecy that the weird sisters had given them. So Lady Macbeth knowing that Banquo's children were supposed to be the next in line to the throne after Macbeth. Since she knew that, and that she is smarter than Macbeth (obviously), and thought that Macbeth was planning to kill Banquo and Fleance. “Thou know’st that Banquo, and his Fleance, lives … There’s comfort yet; they are assailable.” (III,ii,41-43). She didn't want Fleance to die because he is supposed to be the next in line in the prophecy, and thinks that Macbeth is going too far and needs to stop him or at least calm him down. Lady Macbeth I think has woken up from her craziness, and being possessed by the demons. She is starting to feel bad about the murder and wasn't even able to kill duncan herself because he looked too much like her father and looked so peaceful.” Had he not resembled My father as he slept, I had done’t.”(II,ii,16-17). She is also realising how Macbeth is slowly getting more insane, going crazy, and getting more obsessed with his kingship and wants it to be over so he can have his peace. Macbeth has become so paranoid that he would try to kill his best friend just so he can keep the crownship that he didn't even want at first. Also Lady Macbeth can sympathize for Banquo because having your child killed is horrible, and she knows how it feels and does not want someone else to go through that same pain as she had. My last reason is that there was a curse laid upon them that they will never sleep again, I think a lack of sleep is getting in both of their heads and Lady Macbeth wants out and Macbeth to be rid of his kingship so she can sleep peacefully again.
A comment from carters Blog Post.
I think that Hecate being the third murder is a great idea but originally the Hecate scene was written by another writer, so we don't really know that much about Hecate. It could be an option but since that scene was not written by shakespeare we can assume he knew about it while writing the play.