Another wise sage said, "Pain is nothing more than the release of internal weakness." Probably that sage was a career Marine or something, tho...
I had the precise same katate tori sankyo injury inflicted on my right wrist, but from moroti tori. Not fun. Actually I yelled my head off, but the 3rd-Dan instructor who did this told me that I need to tap-tap, since any other response would probably get ignored.
I could not tap, as morotetori had my other hand tied up in there. I jsut collected my squished wrist, looked at him and went, "Huh."