Checkout the Charts page on our samples application.
There is a Custom Value at the top of the page which is zero (0) by default. If you put in a value in there, the Pie Chart changes dynamically (value appears under April).
The underlying data chart directive is tm1-ui-chart-value from which you can just push a value from ng-model. You can then put something for the Others like:
So what is left now is how to get the top 1, 2 and 3 for the above example. Or do you already have this? If not, two of the many possible ways that you can setup a subset to have these three(3) could be through MDX via a combination of TopCount and Filters at the minimum or through brute force with TI.