Figured it out!
joystick_port_0 = xinput controller #2
joystick_port_0_mode = joystick
joystick_port_1 = xinput controller
joystick_port_1_mode = joystick
joystick_port_2 = xinput controller #3
joystick_port_2_mode = joystick
joystick_2_south_button = action_key_shift_right
joystick_2_lstick_left = action_key_x
joystick_2_lstick_right = action_key_z