What about considering a combination of all three?
Macbeth himself possesses an ambition and an impatience which, if given sufficient rein, will be the instruments of his downfall. The witches provide the prophecy which, when the first part is fulfilled and he becomes Thane of Cawdor, causes him to consider, briefly, murdering Duncan. He debates the idea, at this point a fallible but essentially moral man who is disgusted by the 'horrid image' of murder (and remember this is not just any murder, but regicide) and decides that 'if chance will have me king...chance may crown me' without his having to intervene. When he learns that Duncan plans to make his eldest son Malcolm his successor, he becomes tempted to pre-emt fate again, but still he is aware that these thoughts are evil.
It is of course his Lady who actually prompts the murder, and is shown to have far fewer moral scruples than her husband. Undoubtedly it is she who influences him most at this point in the play, because she plays on all his weaknesses, accusing him of not being 'man' enough to kill the king.
Once the king is dead, his guards framed for his murder, and Duncan's sons gone into hiding, Macbeth is named king. But now, the Witches' prophesy becgins to haunt him again - now he must deal with Banquo, whose heirs are to be king. When he sees the Weird Sisters again, they warn him about Macduff.
From here on, Macbeth's tyranny has escalated, and he himself organises the assassinations of those who stand in his way (both botched to an extent - Fleance escapes, and Macduff's wife and children are killed, but Macduff is absent and thus survives.)
I suggest that 'influence' is a fluid thing as this complex play develops. Good luck!
It is a combination of the three.. However you have to remember, its Macbeth's ambition that actually spurs him on. You can say it was the witches who told him of the possibility of being king... They don't mention anything about killing though.. After Macbeth hears the prophecies he immediately thinks of killing. Therefore that shows that he was thinking of killing before he hears the witches predictions. Then we see his conscience as he explains to Lady Macbeth he will not kill Duncan. That is why, even though it is a combination of all three, Lady Macbeth is the main reason behind Macbeths influence. She convinces him to kill Duncan out of self gain and in her own opinion, so that he could be king. So who knows, maybe Macbeth didnt need to kill... Lady Macbeth though, is the main reason he did.
I believe that the best answer is a combination of the three. However if you have to narrow it down you could build a good case for any of the three. Lady Macbeth was such a force in his choices. When we first meet the two we see them with a loving relationship (the letter). We see how he depends on her and values her opinion. We also see her manipulate him into killing Duncan. Somthing he would not have done without her pushing. "if chance will have me king, why, chance may crown me" Lady Macbeth's primary manipulation of Macbeth is through his adoration of her. She accusses him of being less than a man , "When you durst do it, then you were a man" because of their close relationship these inferences create just enough of spur to move him in that direction. Of course this leads to the the counter arugment that very soon he was just as merciless and that he had the propesity for evil because of his ambition.