Begin with your key light which is your primary light, pointed downward at the subject between a twenty and forty-five degree angle, on the axis of the camera to your subject. This is generally the brightest light in your setup, but will only define one side of the subject's face. The subject's shadows should fall out of the frame to retain your shot's illusion of reality.
The backlight, or "rim" light, is used to give shape and definition to the face. This must be placed on the opposite side of your key light, either at the subject's level or a bit higher, depending on the mood you are aiming for. The definition of the subject will distinguish him or her from a blank background. This will allow you to shine another light on the background, if desired, to provide even more depth to your shot.
Your third light, the "fill" light, will alleviate the problem of shadows created by your key light. The fill is usually a softer, more diffused light than your key. You might try this without an actual light, by bouncing or reflecting sunlight onto the subject. If shooting inside, bounce a light off the ceiling or opposite wall, creating a soft blanket effect that will remove the shadows of your other lights while providing the rest of the light needed to get the shot. Try different "fill" setups to achieve different moods, as digital video is capable of picking up small amounts of light.
If shooting with minimal light, set up your back light and reflect the light back onto the front of the subject with a reflector board (you can make one out of cardboard and tinfoil). Now, your back light is also your key light. Use another smaller light as your "fill" and you have a complete set-up for shooting medium shots to extreme close-ups.
If on a tight lighting budget, buy LED light bulbs. If shooting indoors, you can simply switch out your current bulbs with LED's; thus, keeping with a motivated lighting scheme. They will remain compact and hidden from the camera's view while providing an even, diffused glow on your subject. Again, because you are shooting on digital video, the camera (depending on your settings) should be able to pick up its incandescent glow.