It’s called a diode
You feed the center third brake light with two diodes
One feeding from each right and left lights
So if either side brake light is on, the center is on as well.
Identical diodes are almost always good to put in parallel to increase load capacity.
So pick up a handful of 1N400x or similar for about $1 and keep them around the shop
Think of them as check valves for current.
1N400x is the family, specific numbers are like:
1N4001, 1N4002, ...1N4006 etc
They are almost all rated for 1 amp each and the last digit is the reverse voltage capability
The end of the diode with the bar marking would be towards the center 3rd brake light on a negative chassis vehicle in the circuit I described.
-s
