The first witch predicts that Banquo shall be "lesser than Macbeth, and greater". This first prediction is common of the doublespeak used by the witches. Their words ring like a paradox, an equivocation, but close analysis of events later in the play proves the truth of their prediction. Macbeth becomes king and therefore Banquo becomes his subject - he is "lesser" than Macbeth. Banquo, however, retains his integrity and the respect of all who know him, unlike Macbeth who, through his tyranny and ruthless blood-thirst, loses the support and respect of those who were once close to him. They turn against him. Even though Banquo dies, his memory is that of a noble and loyal gentleman, whereas Macbeth is despised and seen as the canker destroying Scotland. In this sense, then, Banquo is "greater" than Macbeth.
The second witch says that Banquo would be "not so happy, yet much happier". Once again, paradox is used. Banquo would obviously not be happy for the death of his king (Duncan) saddens him later, whilst for Macbeth it brings the crown. Macbeth feels no remorse. Secondly Macbeth has Banquo assassinated, so Banquo is therefore once again, "not so happy".
What does make Banquo "much happier" or more fortunate than Macbeth is the fact that in death, he is at peace whilst Macbeth cannot sleep. He becomes steeped in blood and is paranoid, suspecting practically all those around him. He is haunted by the murders of Duncan and Banquo and can therefore not enjoy a peaceful rest. He suffers relentless torment, is constantly on his guard and is overwrought. Furthermore, Banquo had the confidence that his heirs would rule whilst Macbeth would leave behind a barren throne.
The third witch's prediction "Thou wilt get kings, though thou be none" affirms the fact that although Banquo would not be a king himself, his heirs would. His issue would become either be rulers themselves or would be the progenitors of future kings.