type SomeType = [number, number, number]; // Should be an array of exactly 3 numbers

interface IThing {
   someThing: SomeType
}

abstract class SomeClass {
  abstract getThing(): IThing;
}

class ConcreteClass extends SomeClass {
    getThing() {
       return {
          someThing: [4, 2, 2];
       }
    }
}