# /etc/X11/xorg.conf.d/90-touchpad.conf

Section "InputClass"
    Identifier "touchpad"
    Driver "synaptics"
    MatchIsTouchpad "on"
        Option "TapButton1" "1"
        Option "TapButton2" "3"
        Option "TapButton3" "2"
EndSection

# or, depending on HW and distro e.g.

Section "InputClass"
    Identifier "touchpad"
    Driver "libinput"
    MatchIsTouchpad "on"
    Option "Tapping" "on"
EndSection