This explains the process of adding 3 or more extruders.

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Required Hardware:
Arduino 2560 with USB cable
RAMPS 1.4
2xExtrudrBoard
Nema17 motor.
A4988 Stepper driver


Required Software:
Arduino studios 1.8.2 or higher
https://www.arduino.cc/
Pronterface.exe
http://www.pronterface.com/
MarlinFW (Firmware 1.1.6)
http://marlinfw.org/meta/download/