How to merge three web application of different region in single code base

We have a web application. For this web application we have three separate code base for three different region . This was done to deliver region specific functionality. like for USA region page will contain more field then UK region.

Now we want to merge the code base and make it a single application with single code base. application will be like user will select region from drop down at the time of login and page specific to that region will open.

So i need help in designing this application .please suggest how to proceed and what should be design patterns or framework and right approach for such development .