It's not a bad theory, and you support it, but I always felt like Seyton was Murderer Three-we don't even meet Seyton until after the murder of Banquo, yet he seems to be this trusty righthand-man to Macbeth-what better way to earn the king's trust and confidence than carrying out an assassination for him? (In the production i was a part of, Seyton killed the other two murderers after they'd killed Banquo, to make sure there were no witnesses-nothing sloppy-which explains why only the Third Murderer is ever seen again.)

you have tpyos. Just a few, but they're there. I would add a little more detial, but you support your theorys.