Consider what the logical sources of light in the setting are. If you position the lights to give the impression that light is coming in through windows, or being cast by lamps that are built into the set (or are implied, like in a ceiling),  it goes a long way to making the lighting seem more motivated and interesting. A 3 point system is a good fundamental to know, but it's not always the solution for every scenario. If the sun is behind a character, then you can allow the front of them to be darker; it looks less realistic to light up the front of them with another direct light source.