Use the symbol "$" to indicate a light switch. Although this is a dollar symbol outside the world of architecture, on a floor plan it indicates a light switch.
Place the "$" symbol anywhere on the floor plan where you want a light switch installed. For a triple light switch, use the symbol "$3".
Orient the symbol to the wall on which the light switch is going, rather than to the floor plan as a whole. The symbol should be upright when the floor plan is oriented to look at the wall on which it is placed.