diff --git a/src/calc.py b/src/calc.py
index 2af3c17..bcc717a 100644
--- a/src/calc.py
+++ b/src/calc.py
@@ -9,7 +9,7 @@ def add(a, b, third_operand = 0):
     """
     return a + b + third_operand
 
-def sub(a, b):
+def sub(a, b, c=0):
     """
     Subtract some numbers
 
@@ -17,4 +17,4 @@ def sub(a, b):
     sub(8, 3) # 5
     ```
     """
-    return a - b
+    return a - b - c
diff --git a/src/calc_test.py b/src/calc_test.py
index 2911910..2953894 100644
--- a/src/calc_test.py
+++ b/src/calc_test.py
@@ -17,6 +17,9 @@ def test_sub_2arg(self):
         # Make sure 4 - 3 = 1
         self.assertEqual(sub(4, 3), 1, 'subtracting three from four')
 
+    def test_sub_3arg(self):
+        self.assertEqual(sub(4, 3, 1), 0, 'subtracting three and one from four')
+
 
 if __name__ == '__main__':
     unittest.main()