All modern cars are required, by law, to have two working brake lights. Technically, the third brake light must be tested. But if the MoT tester is unsure if it's connected then they should give the car the benefit of the doubt and pass it.

I would recommend getting the bulb/wiring of the third brake light fixed. It's not an expensive job and improves the safety of your vehicle by giving other road users advanced warning when it comes to braking.