I don't think you saw what I wrote properly... you need to use show="y" for the first two, and then show="n" for the final one. You haven't got that in your code examples...
Also, another thing to consider, as a developer, is that the plugin, since the latest version, is using cookies to store the results of the country data. Perhaps in a future version I'll provide a plugin option to allow you to toggle this on/off for development purposes. If you have Google chrome, you'll be able to easily delete the cookie data between your tests.
Also, while this database is fairly accurate, it's not completely 100% and does have I believe a few exceptions.
The database is taken from: http://www.ip2nation.com/ip2nation try putting in your IPs there and see which country comes out. If you're using it exactly as follows:
[CBC country="gb, ie, im" show="y"]England[/CBC]
[CBC country="us" show="y"]USA[/CBC]
[CBC country="gb, ie, im, us" show="n"]Everyone Else[/CBC]
and on the ip2nation.com website it's returning the correct response for the countries given your testing IP addresses, can you email me with your IP addresses that you're using and your exact results and I'll run a few tests against the code.
Of course, please ensure you're deleting the cookie data for the plugin between tests.
I hope all that helps get it all sorted.