says to use EXT3 for CR10-LCD, ...
I have the same setup. EXT3 is correct. You have a choice of interfaces - either the mono display or the touch screen. You can choose by holding down the control knob for a few seconds, then use the knob to select which interface you desire.
Be aware - my touch screen did not function until I changed the serial port settings in Configuration.h in the Marlin software and re-built the firmware as shown below. Hope you don't have to ...
/**
* Select the serial port on the board to use for communication with the host.
* This allows the connection of wireless adapters (for instance) to non-default port pins.
* Note: The first serial port (-1 or 0) will always be used by the Arduino bootloader.
*
* :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
*/
#define SERIAL_PORT -1/**
* Select a secondary serial port on the board to use for communication with the host.
* This allows the connection of wireless adapters (for instance) to non-default port pins.
* Serial port -1 is the USB emulated serial port, if available.
*
* :[-1, 0, 1, 2, 3, 4, 5, 6, 7]
*/
#define SERIAL_PORT_2 2