How many witches?

I would be surprised if this subject is not an old chestnut here, but I cannot find any trace of a thread, so please forgive this repeat:

Act 4 scene 1, Hecate appears immediately after the famous "double trouble" spell, and she brings with her three more witches. Why three extra? Cannot the first three dance like "Like Elves and Fairies in a Ring," or maybe the cauldron was too big?

I think this scene is commonly held to be a later, post mortem, addition, commonly attributed to Middleton if I'm not mistaken. Who knows what he may have been thinking.