Quaife, Kaaz and Cusco are all well-respected companies which make an LSD for the 3. I personally have a Quaife and love it. Tigermack is planning on getting a Kaaz. I don't know anyone with the Cusco... they make awesome stuff, but Tri-Point did do some testing with it and preferred the Quaife in our car for street use.
As far as the install... it's not easy. I had it installed by professionals with lots of experience working on transmissions. I believe there was a little machining of the selector fork involved to make it fit perfectly.