Third (upper on hatch) brake light working, other 2 dead. Tested left and right bulbs (all4) all working when plugged in third (upper) brake light on hatch. Should I go buy a new switch that's located above the brake pedal?

 

The third brake light should be activated by the same switch at the pedal as the other lights so I don't think that is your issue.

I would start by buying new bulbs for left and right (even though you tested them) since they are dual filament and the center bulbs are single filament.

 

The left and right brake lights have two bulbs each, 1 single filament, and one double. I plugged those, all 4 in the hatch and they light up, that means they're not busted right?

 

The dual filament bulbs are the brake light bulbs. If you plug them into the third brake light assembly, you are only testing one of the two filaments.

Other thing to check are fuses/relays. #22 fuse inside the car includes the taillights. Fuse #16 under hood is for brake lights, but I don't believe that will be the issue as it is for left, right, and center.