Get yourself some modern electronics instead of RAMPS, run it from 24V, and connect the three Z motors in series. Then you won't need a high current driver.
The reason people connect two Z stepper motors in parallel is that RAMPS makes it easy and most people don't know any better. For the types of motors typically used, such as the ones you are using, it makes much more sense to connect them in series. You could easily drive two of those motors in series using 12V power. You might even get away with using 12V power and 3 motors in series, but as this is a new build it makes sense to design for 24V.
The reason people connect two Z stepper motors in parallel is that RAMPS makes it easy and most people don't know any better. For the types of motors typically used, such as the ones you are using, it makes much more sense to connect them in series. You could easily drive two of those motors in series using 12V power. You might even get away with using 12V power and 3 motors in series, but as this is a new build it makes sense to design for 24V.