If you haven't found what you are looking for, the December, 2011 edition of the AMA magazine list a source for the Sopwith Triplane in 1/6, 1/4 , and 1/3 scale short kits. I believe that the plans come with the short kits if you order it. The kits are cut from plans by Jerry Behrens.
That triplane has caught my eye too. I'm always looking at anything that flies on more than one wing.
I have a DR-I that's a bit bigger than 1/5 scale. I bought from an estate sale. I didn't know it until I got it home but it's an ARF. Don't know what ARF it is but it flies really well with an OS 1.20 up front. It's got a 60 inch span. I was told at one time that it was from Fly Boy Models, but when I tried to look them up the web site had been shut down.

Dave