I created the /etc/X11/xorg.conf.d/71-synaptics.conf file with the following contents with
Identifier "touchpad catchall"
Option "TapButton3" "2"
and I can see in /var/log/Xorg.0.log that the option has been set.
[ 91056.468] (**) Option "TapButton3" "2"
The option was merged with options I had set similarly in my 70-synaptics.conf file.
If your device uses a different device or different driver (see the /var/log/Xorg.0.log to see), you should adjust the Identifier and Driver lines appropriately.
Another approach is to disable the gnome mouse settings plugin.
To do this start a terminal with Alt+Ctl+T, and then install the dconf-editor:
apt-get install dconf-editor hleinone
and navigate the tree to org.gnome.settings-daemon.plugins.mouse.
- Finally, uncheck the Active box
A drawback of this latter approach is that no settings you configure in System Settings-> Mouse and Touchpad will be effective.
This approach, from the comments, is from here by way of hleinone.