Not entirely true. I've got one on the large flange Campy hub of my International.

This is the 4 prong version of the magnet ring, though. It's been a while since I had to think about this stuff...

Grabbing the calipers quickly, I see that the 4 prong ring has an OD of roughly 52mm, while the 3 prong ring has an OD of 46mm. I don't think the 3 prong ring works on large flange hubs w/o snipping the prongs off and then zip-tying the ring to the spoke crossing. ... .. or just RTV'ing the ring to the hub. I seem to recall having done that in the past.

Steve in Peoria