Campagnolo made three three arm cranks that I'm aware of. There was the steel Sport, the aluminum Grand Sport, and the Sport Baby. The Sport Baby had a 150mm arm and was intended for juveniles.
The Sport and Grand Sport use the same chainrings. The outer ring uses the same bolt circle as a TA Trois Attaches and a lot of other european cranks. The inner ring uses it's own bolt circle which is larger. Sometimes the rings are riveted together and sometimes they are bolted. Campagnolo even made chain guard for the Sport.
TA made at least two different adaptors to attach six pin criterium rings to three pin cranks. These adaptors are rings rather than three little arms. One of my adaptors fits beautifully on a Sport crank and doesn't fit on a Trois Attaches at all.
It's kind of fun to mix and match adaptors. I took a Stronglight steel arm with the small 5 pin attachment (model 45, I think) and connected that to a Simplex 5 pin to three pin adaptor plate. The plate was connected to Alfa Criterium 3 pin to six pin adaptor arms, and those arms were connected to Simplex rings. Fourteen bolts (not counting the cotter) in one crank and not bad looking.
Leonard Bulger
Marblehead, MA