If I want to become an Investment Banker, would I study Econ, Econ/Fin or Finance?

Any of those are fine. In fact any degree is fine, provided you go to a target uni. Plenty of people in investment banking with degrees in e.g. history, classics, anthropology, biology, physics, engineering, philosophy, politics, as well as those. It just tends to be that people in degrees like economics, business management, accounting and/or finance have a tendency to self-select into applying to those roles particularly. Going to a target uni I expect would be the overriding concern - degree subject would just be whatever you find interesting.