Hi Andrew Hyslop,
Thank you for using our services.
There is a problem with the use of nested master pages within Sitefinity 3.x versions. You will not be able to edit the content place holders from the parent master page inside of Sitefinity. With this in mind the best option is to use 3 separate master pages and create themes for them. A layout type of control is not supported in Sitefinity 3.x - you have to define the content place holders within your master pages.
This approach is handled much better in Sitefinity 4.0. There you are able to define a parent template through the Sitefinity administration and create templates which inherit from it. This eliminates the need to use master page files as all of this can be handled through the UI,
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