I’m developing a component which has the below structure. This system is developed under Multi-Tenant concept.

http://www.a.com, http://www.b.com, http://www.c.com are web sites which are in different domains but they run the same system. Each website have their own template and web pages in their local database. That’s doable.
The system which is ran on these three sites are independent from the mentioned above(templates etc). http://www.a.com, http://www.b.com, http://www.c.com have a one large database where all the system data is maintained. Since the system is developed according to MVC what I want to do is have a single model(multiple models) where all the logic is contained. So if I make a single change in the model it affects the 3 sites.
According to my research, model should be maintained in the cloud. How can I implement such a requirement with MVC(Given I’m using Joomla as my CMS, MySQL as DBs) ? How can I have a single model for all the 3 sites ?
You can put more than one domain on the very same joomla installation without any changes, just add a plugin that’s called virtualdomains which will allow you (for free) to choose template and – thus – menus, extensions and available pages for each domain;
Maintenance-wise this will be easy since all is in one box; but the users will see three sites with different content, layout and urls.
Just a couple of warnings:
1) make sure no scripts or css or other local resources are invoked with the url (http://www.a.com/template/xxx/js/jquery.autocomplete.js) vs /template/xxx/js/jquery.autocomplete.js, the first will work badly with cache.
2) handle any intricacies such as same-url on different domains with .htaccess; use a sh404 or similar component to manage the SEF urls.