When we operated the types, one crew roster flew the A340-300, A340-500 and A330-300 at the same time. However, there were Transport Canada restrictions on maintaining competency ... namely if you had not flown one of the three in over 180 days, you had to go back to the training centre for simulator.
Also, when going from the A330/A340 to the A319/320/321 or vv. you did not require a full course, just a so called "short course" as the family similarities were there.
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!