Very creative concept! I like the idea of these three villains getting together. And the hints to their stories in the wardrobe, mirror, broom, map and tornado are great. I feel it is a little stagnant, however. Explore illustrating a different moment in time--like the queen arriving through the mirror. Same with the White Witch through the wardrobe. They each could be bringing something for the tea party--like heart sugar cookies or Turkish Delight. At the very least, make them more animated around the table. Right now it is three similar shapes just sitting there in similar poses. And the poses don't match their personalities. Skewing the perspective helps make it less stagnant, but it falls short of the potential a different moment in time would have.
Before working any more perspective I suggest doing a bunch of thumbnails with the witches and objects in different positions and different moments in time. If you are set on this composition, the back wall is off--especially where it connects to the left wall, the table is too low making it look like it is further back than it should be (the characters legs should be able to go underneath the table...), and the legs of the mirror should be straight up and down, not leaning back.
This is definitely an idea worth exploring and finishing! Can't wait to see more!