Which framework or design patterns to use

I have 3 web applications of 3 different regions running in 3 different server. These 3 applications are more or less same .it has some region specific customization.say for member page if user is from gulf then we will show 5 fields, if user is from other region then we will show 10 fields and so on. So we are managing three different codebase for three different region. Say these 3 applications is devOpgulf, devopUK, devOpUSA.

Now we want to merge these applications in one code base.thought is that we will provide a drop down at the time of login where user can select the region gulf, uk, usa and based on that user will be logged in to a particular region and we will show the region specific page.