I have been looking at the picture of this setup, trying to figure out how the system works.
I think that when you pull the trigger on rifle #1 (middle), it fires and sends its gas to cycle and fire rifle #2 (bottom), which will do the same with #3 (top), which will then send its gas to cycle #1.
If you keep your finger on the trigger on rifle #1, the cycle would then repeat until there’s a jam or the magazines are empty.
Presumably all rifles must be full auto. If you note rifle #2 and #3 they have no triggers and auto sears.
Rilfe #3 doesn’t have a charging handle.
Gas timing could be a bit of an issue as well.
If you were using 30 round magazines in all rifles, I wonder how long time it would take before they were empty?
For sure, you would have brass flying in all sort of directions.
Unfortunately I don’t know the source of the photo or who built it, but I will be happy to add information and credit.