How to configure touchpad two fingers tap as middle click in ubuntu 19.10? Libinput is used

You can add

Option "TappingButtonMap" "lmr"

to the touchpad section of /usr/share/X11/xorg.conf.d/40-libinput.conf file like this:

Section "InputClass"
        Identifier "libinput touchpad catchall"
        MatchIsTouchpad "on"
        MatchDevicePath "/dev/input/event*"
        Driver "libinput"
        Option "TappingButtonMap" "lmr"
EndSection

lmr means "left/middle/right". By default 3-finger tap is for a middle-click (lrm).

It will permanently set this option (after a reboot). You can also test it by setting a xinput option:

xinput set-prop 10 288 0 1 

This should have an effect right away, but won't survive a reboot.