I have been using the dual version of the Allcam monitor bracket for a few years now, and I love it - no problems with 2 x 24" flat screens. The height is a little tricky to adjust but the mobility is good horizontally and tilt is available without an issue. It supports 3 screens and beyond (it is modular).
It is a desk mounted bracket and it supports weights in excess of 13lbs, so it seems to fit your requirements.
The invoice I dug up tells me that in 2011 I ordered the following from Allcam directly:
- 2 x Long Arm for Allcam MDM0X series LCD Monitor Desk Mount Bracket
- 1 x MDM05 Multi Screen Desk Mount Bracket
That came to a total of just over €100 thanks to high taxes and delivery charges to Ireland, but still not excessive.
You would need 2 of the long arms, one short and MDM06 instead, but it's pretty modular, so it's really up to you. Here's a link to the Amazon (UK) version of the MDM06 with decent pictures and an idea of price - I would have used Amazon when I ordered but it was not an option.
I have moved twice since I ordered and had to disassemble and reassemble each time - it's a little tricky if space is tight or you have a tight fit with a wall (but you mention you don't) - but otherwise it is solid, and I have never had any concerns about stability.
While searching for a decent shot of the triple set up I spotted that Allcam have a guide up on Youtube showing you how to set up the triple monitor here so you can get a decent idea of what's involved and how it might look.
As an aside: this was one of the best quality of life things I ever did - I highly recommend it if you can manage the fitting and your desk will hold up - it frees up an insane amount of space.