Yvonne, first make sure the correct bulb is installed (the brake light bulb is a two filament 21/5W), take out the old bulb by pushing in slightly and turning it (I believe) to the left to bypass the locking design of the socket. Take the new 21/5W bulb and install it into the socket by sliding the pins down the slots in the socket. Then turning the bulb (I believe this time to the right), into its locked position (do not force the bulb into the socket, it should go only one way since the pins on the base of the bulb are offset from the end). If you cannot get the bulb to turn after sliding into the socket you might have the bulb 180 degrees out. There is a 50/50 chance it'll go in correctly every time. IF the base is damaged, your going to have to make sure the bulb is correctly inserted (if someone else installed a bulb wrong it could have damaged the socket locking pin locations.

About your third light situation, you mentioned " third light does not work but when I unplugged it and used my volt meter it lit up ", what lit up? The third brake light? If it lit up correctly when you used a voltmeter, maybe the connector is damaged in some way. That may be a lot easier to fix, than sending it in somewhere to be repaired. Appreciate a more detailed description of the third light issue, ok?