In a two-light set up the key light is generally placed above the camera and at about 45° angle to the subject. For flatter, less directional lighting, you might want to place it above and directly behind you camera, with a diffusing filter in front of the light.
The fill light is placed opposite the key light to soften the shadows. Either the positioning of the light or the relative wattage of the two lights can be used to make the intensity of your fill light less than that of your key light.
For portraiture, you may want to use one or both of your lights "bouncing" from an umbrella. This creates a soft, diffused light which cast fewer unflattering shadows than direct lighting.
|Your third light can be uses in a number of ways. In portraiture, it can be directed at the subjects hair to add sparkle. Make the intensity of the hair light slightly grater than that of your key light.
It can also be uses as a background light, to separate subject from the background. Try this effect when shooting on a small set, to add the illusion of more space.
Aimed directly on the background, you can brighten your shot, add separation between the subject and the background, or use color effects gels on your background light to create any color background you like.