Check your MIN frame rate when running triFire. While it is true that your Max FPS will drop slightly because of the imperfections of scaling with 3 cards your min frames should be much more rock solid. If your MIN Fps drops below Xfire that is when you should be looking for bottlenecks or driver issues. BC2 would be a good test as it will use more than 4 threads. So if you have 40 FPS with triFire and it stays at 40 fps even at the most graphically intense areas you are doing well.