From what i've heard being a mechanic, if your car is fitted with three from the factory, you need to have three on there, if you fit a spoiler you need to be able to see the original one, or get one with a brake light in it, and the reverse if you remove it, if it has one fitted to the spoiler you need to have one in the windscreen...
But I dont have any written laws on that, and cbf looking for them!! just ask the local police, or rta for the requirements on brake lights....
Having said all that ...
I have taken the bulb out of my centre brake light in the wind screen, and put a sticker over it - got away with it for like 6 months, but then got sick of wondering if i'd get done or not so I put it back in and ripped the sticker off!!