Unfortunately, the answer to your question is that there IS no definite answer! Hundreds of theater companies have produced William Shakespeare's "Macbeth" since it is undoubtedly one of the Bard's most well-loved tragedies. While there is no "sure" answer, there are a few things to keep in mind about the presumed appearance of the witches. First, based on the text, many theater companies use women who are significantly older (or who are made up to look elderly) to play the three sisters. The play opens with their appearance in a desolate place; their opening chant alerts the readers/audience to their intent to meet with Macbeth. Their words that "Fair is foul, and foul is fair" (Act I, Line 12) show their creed: what is evil to others is good to them. To mirror their dark personalities, they are usually portrayed as old women in long robe-like clothing. They are hunched over, and, in the rare cases that their faces are seen, they have wrinkled skin, pointy noses and stringy hair. In many cases, however, the witches are shown wearing long robes with hoods that cover the majority of their faces. In some theatric productions, the witches carry crooks or long sticks which they use to stir the cauldron into which they toss a number of gross items to concoct their spells. In one British theater's production, the witches are wearing masks (the link is included below). At times, theater directors will choose to use younger women as the witches, but it should be noted that they are never beautiful. Rather, they are portrayed with messy, torn or unkempt clothing, disheveled hair and in some cases, bad teeth. Like their elderly counterparts, they are often hunched over and have hands that look arthritic.
The scene is foggy and dark. The stage is, in many cases, predominantly empty, except for a cauldron in the middle or off to one side. The lighting in many cases is darkened so that the faces of the witches are not seen by the audience; only their cackling voices are heard. The truth of the matter is that the appearance of the witches can be left completely to interpretation; what they should NOT look like is perhaps easier to address than what they SHOULD look like. To portray the witches as young and beautiful would be inconsistent with the text and the personalities they exude. Sticking with a more traditional "Halloween-style" witch is more than likely a more accurate depiction.
I have included a few links below to show how some authors/illustrators have portrayed them. I find that, for me as an educator who frequently teaches "Macbeth," these seem to be the ones that best represent the text; however, virtually any interpretation that shows them as ugly and evil should be considered appropriate.
First, we must disabuse ourselves of Grimm’s Fairy Tales witches—no bent noses with warts. Next, we must get rid of the pointed hat—those were Puritan and Welsh women, but were later depictions of witches (post-1600); next, we must remember that all actors were men during this time; finally, witches in the Elizabethan era were social outcasts, usually Druidic or at least non-Christian, living in forests and in caves, not welcomed in society because they were thought to cast spells on persons who displeased them for any reason. We are left with older "women," with long grey stringy hair (down around their ears, a sign of madness), never pinned up or coifed, and probably not scarfed (because they would look like men). They should wear long skirts, aprons with pockets (for the ingredients of their concoction), multi-layered blouses, vests, etc., and a “wrap” of some kind on their shoulders, wool or some heavy material, not silk. Bent from age, unkempt, raspy voices, knarled hands. The three witches should be visibly distinct from each other, and one should be the “leader” of the group. Their clothes are old, used, non-fashionable, with leaves, vines, thorns, etc. stuck in them to indicate that they live in the forest.