You have two other main brake lights which are obligatory. I don't think the 'third' light is part of the test other than, if one is fitted it should be operational.
See here:
UKMOT.com MOT Manual 1.3 Brakes lights
Hope I read it correctly. I understood 'unobscured' to mean that the brake light is not partially hidden behind part of the car or by something added to the bodywork/bumper to obscure it.