Add exercises 1, 2 and 3

Activity_3_python/exercise_3/ex3_6_1.py

@@ -0,0 +1,16 @@
+msg = input("Message à coder ? ")
+res = ""
+alph = 'abcdefghijklmnopqrstuvwxyz'
+
+for i in msg:
+    if i == ' ' or i =="'":
+        res += i
+        
+    else:
+        p = alph.find(i)
+        t = p + 3
+        if t > 25:
+            t = t - 26
+        res += alph[t]
+
+print(res)