Add Exercise 3 README file

Contains some explanation on the current exercise
Add Exercise 3 main python file

On some numbers and math
Add Ex3 Study Drills scripts

1. Commenting each lines
2. Custom made python basic calculation ability
3. Using float number
Update README file

Now with the names of the exercises
Merge branch 'feature/ex3' into develop
Update main README file

Exercise 3 is now completed