I can't speak for a triple monitor set-up, I have a quad monitor set-up and a dual monitor set up.
Both of these set on top of my desktop but I have put post supports under the desk going from the ground up to the bottom of the desktop to help support the extra weight.
You can look here and see if anything peaks your interest:
http://shop.easymountlcd.com/LCD-Monitor-Stands_c2.htm
One thing I would take notice of, is how the monitor mounts are: are they setting directly on the desktop or are they "clamped" onto the back of the desktop.
If the monitor mount is clamped to the back of the desktop, you better make sure the desktop is rugged enough to handle the weight- make sure the desktop is 1" thick, or reinforce the clamps under the desktop with metal bars. Something that is 1" wide x 1/8" thick x 4" (or so) long.
I have one question though.... If the monitors are going to be side by side, why would you need any additional mounts to connect them all?
They should set side by side fairly well using their existing stands....