Hi Ulco,
I haven't had a chance to look at your project file yet, but I will do so.
Fleetwood,
I think your TG2 render might be using either an OBJ or a Rock object, because I can see that it's faceted at the edges. OBJ Reader, TGO Reader, Card, Rock and Grass Clump render using one method, while Sphere, Plane, Disc and Lake use a separate method. There are problems with opacity on that second kind of object. As far as I can remember, this was a problem in TG2 as well, but if you're using an OBJ or Rock it should work correctly in both TG2 and TG3.
Matt