added day-3 exercises

exercises/Day-3/exercise-1/exercise-1.md

exercises/Day-3/exercise-1/gaussiansmootherx_test.py

exercises/Day-3/exercise-1/main.py

exercises/Day-3/exercise-2/bitmap.py

exercises/Day-3/exercise-2/exercise-2.md

exercises/Day-3/exercise-2/gaussiansmootherx.py

exercises/Day-3/exercise-3/exercise-3.md

exercises/Day-3/exercise-3/main-joblib.py