Which characters appear to be the antagonists to Macbeth in Act Three?

In Act 3, Macbeth has been crowned king, and instead of allaying his fears and his ambitions, his new crown is only increasing both. He begins especially to worry about Banquo, who becomes his antagonist in this act. He finds himself unsatisfied in his new position as king because he still feels threatened: "To be thus is nothing, but to be safely thus" [emphasis mine]. The witches prophesied that Banquo's children would eventually become kings...

In Act 3, Macbeth has been crowned king, and instead of allaying his fears and his ambitions, his new crown is only increasing both. He begins especially to worry about Banquo, who becomes his antagonist in this act. He finds himself unsatisfied in his new position as king because he still feels threatened: "To be thus is nothing, but to be safely thus" [emphasis mine]. The witches prophesied that Banquo's children would eventually become kings themselves, and that means that Macbeth will be unable to pass his crown to his own children—should he have them—and could possibly mean that Banquo's son Fleance will one day overthrow him. Macbeth sees it clearly: Banquo is now his enemy, and he and his son have to die. Unbeknownst to Macbeth, Banquo himself has also begun to suspect that Macbeth may have had a hand in Duncan's murder. Macbeth sends men to dispatch his new chief antagonists, but Banquo will still manage to haunt him—literally.