During a seminar with Suganuma Sensei he had uke maintain a tightly closed fist while demonstrating sankyo. It seemed like he put the palm of his free hand on ukes thumb and applied pressure. Uke moved pretty fast at that point.
When doing it myself I find this is immediately effective with folks of equal or lesser strength and sporadically effective with those who are physically stronger.
We don't practise this way alot, however, I could see it becoming quite effective regardless of ukes size when studied more often.