The witches, in Macbeth, play a major role in the development of the plot. They are introduced first which goes some way to preparing the audience for their scheming and believability.
Banquo may be intrigued by the witches and their prophesies but he is conscious of the evil nature of the 'weird sisters' and is not prepared to accept their prophesies without question
The instruments of darkness tell us truths, / Win us with honest trifles, to betray [us
In Act III, Banquo almost gloats that Macbeth has his way in terms of the witches prophesies, as Duncan is now dead. He does not, however, trust Macbeth - whom he considered his friend.
He has come to the realization though that, if Macbeth
..hast it now - King, Cawdor, Glamis, all
then perhaps he can expect some benefit after all and his sons will indeed be kings. He is contemplative and even hopeful for the future in the face of the current dire situation. He therefore seems to resolve to please Macbeth and show his loyalty to ensure his position and any future for his sons.