It's quite handily the best of the three but by no means a good movie. There were fun successes and painful failures. I really bought the transition from grey area to bad guy, all of which is due to the solid work of the bigger bad guy. Any scene with emotions was horribly overwrought but I love a good swordfight and that's pretty much all I wanted to see, expectations having been decimated by the first two.
I think that was the major pitfall. There was material for one movie (this one) but somehow three got made. Also, the director thought it would be a good idea to take his 1930's update and bring it back to the 1930's. That's the only explanation I have for the dialogue and delivery.
-Lava = cool
-Frankenstein = unnecessary
-The subplot on the fall of democracies was actually well done. It's probably the best part of the first trilogy. Probably because it's the part that was done quietly (well, quiet for these films).
-The turnover rate of technology is so much higher in the first three than in the second three!