Re: adding a third toon
You can do that but it is a bit more complicated than just throwing a few bolts in, your first concern is you have 18'' tubes, which I think are really 19'', never heard of 18'' tubes, anyhow, finding a center tube that is 18''-19'' diameter is going to be a challenge, normally center tubes are quite alot larger, normally 25''.
you could use one of them but it would effectivley turn the outboard tubes into outriggers and I think the boat would be horrible in any kind of wake or wave situation since it is ridding on the center tube but there are companies that do that to get a banking boat.
Once you find a tube you have to be sure it will fit under you boat with the engine in the right place and all the brackets line up.
SO yes it can be done but there is a good bit of math and alot of tape measure reading invloved.