How does Macbeth try to convince the murderers to target Banquo?
Macbeth is, at this point, utterly convinced by the witches' prophesies and he fears mostly their prediction that Banquo's issue would be the future kings of Scotland. It is for this reason that both Banquo and his son, Fleance, should be assassinated. For this purpose he has summoned these murderers, not killers or assassins in the normal sense, but soldiers who have killed many in battle.
Macbeth had consulted with the assassins the previous day and now uses very persuasive and direct language to convince them to carry through his instructions. Macbeth tells them:
That it was he in the times past which held you
So under fortune, which you thought had been
Our innocent self:"
He tells them that Banquo had withheld their promotion. The men were probably entitled to a higher rank, but had been denied by Banquo's refusal. Macbeth says that the men had believed that he (Macbeth) had been responsible for this injustice, but that he was innocent. The men had been misled and Banquo was the one who had made false promises and denied them.
It is clear that Macbeth had already spoken about this to these men and this second speech serves as a reminder of the injustice and dishonour they suffered. Macbeth uses rhetorical questions, asking the men whether they are so patient and holy to let Banquo and his issue get away with such a grave injustice, a foul act which put them at a great disadvantage, so much so that they suffered because of it. Macbeth wants to incense them so much that they would murder to regain their honour.
When the first murderer says, "We are men, my liege", implying that they are honourable and courageous and would not run away from a challenge, Macbeth uses allusion, saying that in spite of its nature, a dog will still be called a dog. Macbeth refers to their manhood, that if they have a station in life, they should claim it and he will give them opportunity to do so, unlike an ordinary man. He will love them for carrying out his instruction and getting rid of their common enemy, for he cannot live comfortably knowing that Banquo is still alive.
The murderers state that they are tired of their suffering and would do anything to bring an end to it. Macbeth seeks their assurance that they agree that Banquo is their enemy. The men do so and Macbeth tells them that he cannot be suspected of having had Banquo killed, since Banquo's allies will turn against him. "This business" as Macbeth calls it, must be secret.
In the end he tells the murderers that he will provide exact details where Banquo is to be accosted and that Fleance should also be killed. There should be no loose ends.
The murderers state that they are "resolved". They have made up their minds and will go through with the datardly deed.
In Act 3, sc. 1, Macbeth tells the murderers that Banquo was their enemy; that Banquo had made their lives difficult. Macbeth tells them that Banquo deceived them, used information against them, and did other things to generally make their lives unfortunate. "...Know / That it was he, in the times past, which held you / So under fortune, ..." Then he uses the tactic of suggesting to them that they aren't real men if they don't get revenge by killing Banquo. Also that if they love Macbeth and are loyal to him, they will do his bidding and that by doing his bidding, the murderers will be even closer to him.