The three Weird Sisters seem to mean that people who look "fair" are often deceiving, and they may be referring specifically to Lady Macbeth. As far as "foul is fair" is concerned, they may be referring to themselves, because they are horrible-looking but are offering honest advice. Duncan is speaking of the traitorous Thane of Cawdor when he says that there is no way of judging a person's character from his appearance. This is at a time when he is about to be murdered by Macbeth and Lady Macbeth, who are welcoming him with the utmost cordiality. Lady Macbeth is especially good at hiding her true character and her secret thoughts. She is flattering the King outrageously while secretly planning to murder him that very night.
Is it really true that there is no way of judging people from their appearance? I don't think so, even though wicked people will always try to hide their true natures. We should always be a little suspicious when people, especially strangers, are too friendly. Perhaps Duncan should have been a little more suspicious of his host and hostess, especially after being double-crossed by the Thane of Cawdor. Duncan should have asked himself why Lady Macbeth was acting so exceedingly joyful to see him and to have him as her overnight guest. The audience, of course, knows that her behavior is completely phoney and can appreciate how good she is at dissimulation. But Duncan ought to have been a little smarter. After all, he has lived a long time, and he is the King.