White or pale shades of the gray or blue. If you choose white, you can add those colors in the room that compliments with whatever furnishings the room is intended for. Silver or nickel would also pull those colors in. Using print fabric would make it interesting instead of solids or an area rug that picks up the colors of the other two rooms.
I would choose a soft pale yellow to go with this third room instead of using too much blue and gray. Strong colors bounce on white which would make it look very pale gray anyway as blue and yellow are complimentary colors.