Brake lights

I have most of the wiring figured out for my rear lights. Now, how do I wire the brake lights, when I apply my brakes will my turn signal not flash. For me the easy way would be to just run a third light for my brake light. It looks to me that automotive flasher, has 2 prongs, should be run like this, does it over ride the brake signal and keep just one of the lights flashing, will both lights flash if I hit the brakes?