Iron Dragon ran all 3 trains this past season at CP. I think the reason why they don't run 3 trains per side on Gemini anymore is cause they don't need 3 trains as the ride isn't as popular as it was in the 70's and 80's and early 90's. Corkscrew usually rotates trains every year.
Gemini and Corkscrew stopped running three trains because of gates and seatbelts. The station time increased and caused stacking issues.