You should check this place. They have a 3 monitor stand mount that uses a curved bar. They are reasonably priced. http://ergomart.com/lcd_monitor_stands/lcd_monitor_stand.htm I have 2 quad mounts LS 1528 Q's but I only use the upper mounts and just rest my other screens on my desk. You can use the mounts I have and just mount 1 monitor on one mount and 2 on the other for a 3 monitor mount across the top. Here a pic of my setup. Top row are all 4 20" across top and 3 monitors on bottom including a 30".
I bought two quad mounts Ergotron DS100 . The cheapest price I could find was from provantage at 240 a piece. Pretty good price, other places asking 280-350 a piece. I have six 21.6 samsung monitors and two 19 inchers.
Below is the link. They also have a ergotron lX triple for 175 and change. http://www.provantage.com/ergotron~110ERGT.htm