Upgrade Ender 3 with Diamond Hotend
The RUMBA boards run up to 36&nbsp;V, so reusing the (24&nbsp;V) power supply from your printer should be fine. As the 2 extra steppers do not take much extra power (note you are ordering only 1!), your supply should be sufficient. Note that a 12&nbsp;V heater cartridge is not going to work on a 24&nbsp;V board (you'll get a massive overshoot and probably over-temperature warnings and printer shutdowns).