Need help with multiple custom menus

I’m in the process of creating a site that is basically divided into three separate sections or sub-sites for Service A, Service B and Service C. There is also a simple landing page with links to each of the three sections. Each section uses a custom page template with a custom header.php file that calls a custom nav menu for that section. It might be a clunky solution, but it works. The problem is that I would like to have some content that is common to all 3 sections, but these common pages and/or posts need to be associated with the correct nav menu when they are called to different sections of the site. Any ideas?