Same Navigation Menu Across DIfferent Themes

I have three WP installs each with different themes in different directories of the same site.

I am trying to make a horizontal navigation menu that is on the very top of the screen and is the same for all sites, but cannot figure out how.

I have tried creating custom menus and using various menu plugins all to no avail.

I know I can call custom css classes in the custom menu, but am not a great coder and can’t get it to work?

Can anyone suggest either a workaround or help me figure out how to call classes to my custom menu and also how to place the code for the header?

Thank you