With 2 Cards in CF you only gain at best 30%,it may be as little as 20% so i was told when i went CF. I think to go Tri fire would be a waste of money imo,the card will not get to anywere near its potential,well your second card wont to be honest.Unless you plan on running resolutions above 1920x1200 i would not go tri.I will not go CF again,i would rather save a little more and get a better single card,at least you know that you will be using all of it.
CF is better than "30% at best". At best, I've seen a few games with 100% scalability or strangely enough, more than 100%, but I've sen some with 0% (if CF is not supported).
Check this out for CF scaling: http://www.xbitlabs.com/articles/video/display/radeon-hd5870-cpu-scaling_4.html#sect1
Keep in mind, that if your CPU is holding you back, it won't scale as well, which is why the higher resolutions show higher scaling as they put more stress on the GPU in comparison to the CPU.