The Workrite Sequioa can handle 3 monitors with a physical width of up to 23". Your 24" monitors may not be that wide, since the 24" is a diagonal measurement.
http://www.workriteergo.com/products/monitor-arms/sequoia/
The Humanscale ParaFlex can support up to three 26"monitors with the long arms. It's a two post system that only offers height adjustability of the full unit, but no forward extension...and it's nto a height you want to adjust all the time. Pay no attention to the list price. Assume you can get it for about 35% off if you buy through a commercial dealer.
http://humanscale.com/products/product_detail.cfm?group=Para/Flex
If you want a triple monitor/single arm unit to act like a single monitor/single arm unit, in terms of movement, you're pretty much limited to 18" monitors (at largest). Also, the more weight you have on each desk mounting location (stand, edge clamp or grommet clamp), and the more you add movement and asjustability, you run into possible issues with shaking/boucing if you type with fervor.