You need to give your sidebar container a unique name and you need to reload your skin before it's seen ( admin > global settings > skins > reload )

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Ye gods, that was quick!
Tried both those things. Just called the third sidebar in the style of the others ie sidebar3 and reloaded. Even working on a skin before uploading it still won't show up and on one occasion after installing it showed "no containers were detected"
Aha - fixed it. Stupidly I'd neglected to rename the div Id in sidebar_3 file. Thanks for making me look again.

I had the same issue with my new skin, and managed to find this post.

Wow, that is so cool.