I agree with Milly's Mum ... keep the cats out of the way of the Dog in a room they can feel safe in. Provide them with all they will need (litter tray, food, water, toys, etc). It might also be worth in investing in a feliway plug-in to put in the room with them just to help keep them a bit calmer (even if they don't see the D O G they will know it is in their house).