I have a created a web component, let's call it
bower install <username>/element
--- lots of compoenents
--- including b-2 and c-3 added manually by me
---- in order for the demo to work locally, but since they
----- aren't in bower.json, this won't work eventually
Publishing multiple elements in a GitHub repository is exactly the same as publishing a single one. The b-2.html and c-3.html elements in your case should be placed besides a-1.html in the directory:
bower.json a-1.html b-2.html c-3.html
Unless specified in an ignore section of the bower.json file, these should be installed by bower.
In my point of view, the main section of the bower.json file is here subtle. Multiple elements should be considered as individual which could be used independently. Here could be a main section for your situation:
"main": [ "a-1.html", "b-2.html", "c-3.html" ]
However, if you wish to document your elements with an iron-component-page component, according to the Polymer 1.0 documentation all elements should be imported in a single html page. The recommended way of setting up the components would be a subdirectory for each component in order to write a demo page per component. I did that in a personal project including several elements to define an UML model: polymeria-uml.