feat(part3): add answer for Exercise 3.

part_3/exercise_3/src/readme.md

+## Answer:
+For the requirement:
+> - The program could potentially support more shapes in the future than the three currently listed (triangle, quadrilateral, circle).
+> - More calculable operations should be supported than currently (area and boundaries).
+> - One generally desired feature is that the program can better utilize the support offered by the compiler and the language types for describing the operational logic, given that a compiled language is being used (Java, but also applicable to other languages).
+
+- Currently, we have `area` and `boundaries` operations in all shapes (triangle, quadrilateral, circle) and the formulas of the operations are based on the shape type (e.g: the area of the circle is different from the area of the triangle).
+    
+    -> 
+    - We have these classes: `Triangle`, `Quadrilateral` and `Circle`. All of them implement an interface: `ShapeInterface` with 2 methods for now: `boundaries()` and `area()`. 
+    - Later, if we have another shape, we can create a new class that implements the `ShapeInterface`. If we have more operations, we should add the new operations to the `ShapeInterface` and implement them in all shape classes.
+    - With this way, the program can better utilize the support offered by the compiler and the language types for describing the operational logic.
+
+- The expected result is a summary of all area and boundaries of all patterns, it means there is something that has same operations (`area()`, `boundaries()`) with `Circle`, `Triangle`, `Quadrilateral` and it is a composite of them
+  
+  ->
+    - We have a `CompositeShape` class that also implements the `ShapeInterface`.
+
+
+Next requirement:
+> - The number of shapes could also vary from zero to an arbitrarily large number.
+
+-> It means the `CompositeShape` class may contain multiple shapes.
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