What's the best way to create "modular websites"? – In other words….

…I would like to have 3 WordPress-based websites (let’s call them website A, website B and website C) that each pull-in certain components (i.e. header, footer, left sidebar, right sidebar and main content) from other WordPress-based websites that are limited to, and setup specifically for each of the components mentioned above i.e. header, footer etc.

The idea is to have a user maintain the content of each component in just the one place (or in other words, they make any changes just once) and the changes are reflected in the 3 websites (A, B and C) – if the component in question is being used there.

The diagram here best explains what I’m trying to achieve.

The following is an explanation of the diagram and my requirements:

o The components mentioned above are each listed in a beige shaded box towards the bottom left of the diagram.
o Each of those components are in essence separate WP-based websites
o Notice that the “header”, “footer” and “content” components (red, purple and cyan boxes, with red, purple and cyan arrow-head lines) are each called/pulled-in for all 3 websites i.e. A, B and C.
o The left sidebar (beige box and arrow-head lines) is called only in website A and B
o The right sidebar (black box and arrow-head line) is called only in website A

Once again, if a change is made to the header component, that change will/must be reflected in all 3 websites.

What would be the best way to achieve this objective?

I’m new to WordPress…so I may not be able to achieve extremely complex suggestions.

Thanks.

Gerard