To all Sentelic touchpad users, here is a small workaround for the missing third mouse button. I often use the button for resizing windows or drop down marked text somewhere. So I really need it often. The workaround is to use the nearly needless menu button from the keyboard for it (between Alt Gr and Ctrl).
First you need to install xkbset for it:
The nice tool named xev (from the package x11-utils) told me, that the button has the keycode 135 for my DE keyboard. Test this out for your layout or for choosing a different key you do not need (pause for example).
sudo apt-get install xkbset
Then create a script file which you make executable and put in the autostart. Fill it with this lines:
Mouse button 2 is the middle one, button 3 the right one!
xmodmap -e "keycode 135 = Pointer_Button2"
Hope this is usefull. Bye!