It occurs to me that the witches are not saying two different things but are saying the same thing in two different ways and that they can only be referring to Lady Macbeth because there is no other character in the play who can be described as "fair." It would not be appropriate to describe Macbeth or any other male as "fair." Lady Macbeth, however, must be thought of as exceptionally attractive if she can have such a strong hold on her husband. The witches, being female themselves, might be especially interested in Lady Macbeth because they are ugly and possibly jealous. The word "fair" should make us think of a beautiful woman. The witches are not talking about picturesque landscapes or using the word as a metaphor for a good outcome in a battle. Lady Macbeth may be called fair but foul because she is fair on the outside but foul on the inside. She may also be called foul but fair for the same reason: she is foul on the inside but presents a fair appearance. There are many fanciful ways of interpreting what the witches say, but I can see only one literal meaning to their words, i.e., that Lady Macbeth is fair and foul and foul and fair. She will be the one who leads Macbeth to his doom. Without her goading and sexual extortion, Macbeth would not have murdered King Duncan. She uses her beauty and sex appeal flagrantly and ruthlessly in order to manipulate her doting husband.
The scene, in its original setting, a stage with minimal scenery, seeks to separate the "real" world from the world of the witches, and part of that effort is to establish the barren, wind-blown Scottish heath, the home of the witches. When they say "Fair is foul and foul is fair," they are contrasting their ideal climate with the climate that the audience, and the other characters, would consider ideal. Paraphrasing: "What is foul to you is ideal, is 'fair' for us; we like 'foul' weather." "Our ideal world, where fillet of fenny snake and eye of newt and tongue of bat can be combined to conjure up visions and predictions of the future, may be ugly to you but it's ideal for us and our leader, Hecate." The phrase also points to the coming imbalance of natural order, the chaos of lost kingships (or thaneships). It shows that this world is out of order, because of the breakdown of the Great Chain of Being.
The witches are practically saying that doing wrong is good and doing good is wrong. I hope this helps you with what you are doing.