Question : What is the difference between Single domain and Three domains version?
Board800 has two components. Server side component runs on Red5 and client side component runs within a browser and is a flash file (.swf). When the flash file (whiteboard.swf) is loaded, then that swf tries to connect to the Red5 server. The server name is hard-coded into that swf file.
When a Single domain license is mentioned, it signifies: after you purchase Board800, you get an option to specify the server domain name. That server domain is basically your Red5 server's domain name. It will be hard-coded into the swf file, and thus the version that you download would automatically connect to your server only.
A three domain version is just that - you would be able to specify three different servers. This would usually be in an environment wherein there's a requirement of running multiple Red5 servers (and multiple whiteboard instances).