Hi Jim. Try Sprint Manufacturing they specialise in Hinckley Triumph parts. I've used them in the past. They do list a complete 3 into 1 system for the Thunderbird Triple.
Have you tried world of triumph, they might have new old stock but it's seven years since I left the company (although I was more on the Honda side of the parts by then) so I can't remember of there were even any there then. There was often shop soiled stuff too. Best of luck.