Why, in Act I, Scene 3, does Macbeth trust the witches instead of Banquo?
The most logical answer to this question seems to be that the witches told Macbeth something that he wanted to hear. When they greeted him by hailing him as "thane of Glamis," "thane of Cawdor" and "Macbeth, that shalt be King hereafter," they stimulated his considerable ambition. But the scene actually unfolds in a more complex way than that. Macbeth is not immediately overjoyed to hear what the witches have to say, and instead looks startled and pensive. Even Banquo asks him at first why he is not happier to hear these prophecies, wondering why he seems "to fear/Things that do sound so fair?"
It is after Ross arrives to inform Macbeth that the thane of Cawdor has died, and that Duncan has given the title to him, that Macbeth seems to accept the witches' prophecy, and Banquo, conversely, seems more fearful. After this part of the prediction is confirmed, Banquo warns his friend that the witches, while truthful, still might be malevolent. "Oftentimes," he tells his friend, "to win us to our harm, /The instruments of darkness tell us truths." But having heard part of the prophecy confirmed, Macbeth seems less willing to listen to his friend.