[ChuckG]

The third - just in case. Helps make if fail proof.

Without the third it would be considered a Herbie Style overflow. Depending on how things are set up, the third may or may not get used at system startup before the siphon kicks in and the water levels off. In my case - it doesn't get used on start up so it's there just in case.

Just in case something happens to plug up a pipe or something of that nature. I left that pipe above water level so that if it ever kicks in it will make noise that I will hear so that I can attend to things.

There's also a little more safety built into the durso standpipe as well. The air tube that feeds it is bent around and kept just below the third (emergency) standpipe. IF the water gets to this level then air is cut off and turns the durso standpipe into a full siphon as well (which handles more water than an open (air sucking) standpipe).