libinput_device_config_tap_set_button_map(libinput_device, button_map);

/* You can choose between:
LIBINPUT_CONFIG_TAP_MAP_LRM -- 1/2/3 finger tap maps to left/right/middle
LIBINPUT_CONFIG_TAP_MAP_LMR -- 1/2/3 finger tap maps to left/middle/right
*/
static const enum libinput_config_tap_button_map button_map = LIBINPUT_CONFIG_TAP_MAP_LRM;