The Sun Sun Rays ultrathin client will do multiple monitors you can get one that supports 2 monitors and you can link a third screen by binding the two sunrays together on the server to give you three monitors.
If you've got a workstation with muliple network cards and graphic cards you could open RDP sessions to separate servers on separate networks across separate screens.
We used to use a Terminal services to a management server that could route to other servers and have muliple TSE sessions across multiple monitors but always used workstations or laptops with two screens to do this.
Don't see the point in having a super spec thin client device that costs as much as a workstation you might as well get a good deal on a cheap workstation becomes more flexible but does depend where and why you're trying to do this