The thing is when you put three cards in a case they are so close together that the cards dont get any airflow.  I have three 7950s in my Thermaltake Level 10 and I have to keep the side off with a box fan blowing at max on it just to keep the top card below 80 C.

Open air rigs are a way better choice.  Either go the milk crate route or build a little thing out of wood.  You might not think so now but you will want to OC the cards once you get it setup and see the hash rate you get out of stock clocks.