Here is the device from Amazon, https://www.amazon.com/dp/B00NUNDVCI?psc=1"
How would you go about wiring it with diodes to make all three lamps work with the flasher? If I'm thinking what you're thinking is get power from third light and run wires with diodes to brake lights and tap there?It will if you use diodes. I think your best bet is a basic flasher system for the 3rd brake light.