Welcome to the Q&A Forum

Actually what you are asking is quite easy. First of all you must create as many GWT profiles as are the subfolders you want to geotarget. When those profiles have been created, you must go to "Configuration/Settings" and choose the country you want the subfolder to geotarget. in the case you want the main root of your domain to target "all the world", then I suggest you to implement the rel="alternate" hreflang mark-up, in order to not having the "global" version of the site (or any other version with a strong link profile), outranking the other versions of your own site.

Using hreflang is ideal for your situation with all your sites are in English.  You may wish have have one site as a the default english language site and the other two targeted to specific countries (in this example UK and US). Here's example of hreflang implementation to place in the head of your pages. I would tread very carefully if using canonical with hreflang as Google no longer recommends it, although it doesn't rule it out comepletely either! Take a look at [this link](http://www.ingenuitydigital.co.uk/News/google-no-longer-recommends-relcanonical-hreflang-tags-1820.aspx) to find out more.