Having trouble with getting thingiverse to render the stl file though. If you can't pull it from there, pm me and I can send it to you. This is a mod of my 4 fin jig. I have not specifically tested the 3 fin version, but would assume it works well as the 4 fin does.
I'm wondering if Thingiverse updated as the upload process is a little different than a couple days ago. If you click on Thing Files, then BT55_3fin_v1.stl, you should be able to get the file. If that doesn't work, pm me and I can either email it or put it on my ftp.