You can add the 3rd light via the serial number found on the base of the bulb. In the app where you added the other bulbs, there is an option to add a bulb via the serial number. If that fails to work, you could possibly have a defective bulbs. Keep in my the bulb needs to be with in range of the hub or it won't work either. I think it is approximately 100ft, but if you have walls or other obstructions, it will alter that distance.