Hi
I converted a C15 to 12volt and it is probably the same as a triumph set up if it is a three wire alternater then two of the wires are joined up to bring two sets of coils in to play.
Anyway have a look at this site(link below) and click on the convert to 12volt link on the left hand menu.
Hope this helps
Richard
http://www.bsa-c15.org.uk/