Pick a paint color that is a darker color than the walls in the rest of the rooms. Use it to create an accent wall on one of the shortest walls in the room. The dark color visually draws the wall forward, thus making the rooms look less narrow. Another option is to paint each room with a coordinating paint that is two or three shades lighter or darker than the one it connects to. This creates visual continuity, yet makes the space seem separate.
Position a couch, room divider, sofa table or even a freestanding shelving unit so it creates definition between the kitchen and the adjacent room. Avoid placing the decorative piece in the walkway.
Expand the width of a room optically by adding a large mirror or a grouping of mirrors on one of the long walls. Provide even greater impact by placing the mirrors opposite windows.
Trick the eye into thinking the rooms have more of a square shape. Paint the long walls a light color and use a darker coordinating shade on the short walls.
Position furniture so it doesn't face toward the length of the three rooms but toward a central focus within each individual room. For instance, if there is a fireplace that is the center of attention in one room, position the sofa and other furniture so they are angled toward this area. The goal is to bring everyone's attention on the focal point, rather than the long narrow space.
Form conversation areas by utilizing area rugs, end tables, lamps and upholstered chairs to create intimate areas. Long, narrow rooms often seem like a warehouse space; the seating areas provide cozy places to gather. Another option is to set a small table with chairs in an area to play card games or build a puzzle.