Here are some general tips to make your backlight look great in your three point lighting setup.
Three Point Lighting
Three point lighting really depends on the unity between three specific lighting positions. They key light is the most dramatic one of the subject's fact that defines the front of them. The fill light, which often comes between the center of the face and the opposing side to that struck by the key, is soft and will fill in the gap. This can often be done with a white card brought close to the face to bounce light back onto the skin.
The third is the backlight that is pointed from behind to the back of the subject's head. It is the backlight that is the most disputed in the three point lighting set up as different people want dramatically different things with it. Here are a few tips for making the backlight work for you in three point lighting.
Backlight to Cut Out
The key purpose of a backlight is so that the subject is removed from their background. The backlight acts to cut them out of the frame and make them truly stand out. To do this you have to see the backlight openly and not just have a hint that it is there, such as with the fill. After each set up check your monitor or the LCD display on your camera to see how it looks. You should at all times be able to actually see a bit of the light on the back of head and shoulders. This does not mean that you are making a creative choice about how dramatic you want it, but instead just following the basic principle so that it actually performs the task for which it was intended.
This conversation leads up to exactly how dramatic you want the backlight to be. Many people think that using too hot of a backlight on someone's head will be too old fashion and makes them appear angelic. This may be true for most news productions, but in a more theatrical interview or dramatic situation the more backlight is usually the better. This means that you can usually title the backlight more in the direction of the person's body than off access. Do this within reason, and if the backlight begins to make the subject too dark you can begin to off tilt the light position.
Not In Frame
Positioning the backlight is important, not just for image control, but simply in terms of technical positioning. If you get the backlight too in line with the framing of the image you can end up making it visible or getting camera flare. You always want the lighting to appear, at least in context, invisible, even though you can see it obviously. This means you should not draw attention to the actual equipment of your lighting.
The height of your backlight is often going to be a major concern in three point lighting. The rule of thumb to follow for interviews is to keep the backlight one to two feet above the subject's head. For dramatic use of three point lighting you are much more open, but you still want to keep it above their position. If you want to lower the backlight you are really going to have to completely readjust the camera position.