Cant say why they had the 3rd ring. Unless there are some documents around there is no way to know for sure . However we can speculate. My first guess would be controlling the fuel charge as the piston pass over the ports. Or possibly to improve crankcase pressure. Or maybe is to stabilize that heavy cast piston so it doesn’t rock in the cylinder. Or to keep oil between the upper and lower rings for better lubrication. This is early in engine design. Things were being tried only to be dropped as they didn’t really work as intended, cost didn’t justify the benefit, but most likely, same as today, engineers have to keep coming up with something different, no matter how well something works, if they want to have a job.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.
