diff --git a/src/calc.py b/src/calc.py
index 2af3c17..5c8ae15 100644
--- a/src/calc.py
+++ b/src/calc.py
@@ -18,3 +18,6 @@ def sub(a, b):
     ```
     """
     return a - b
+
+def sub(a, b, c=0):
+    return a - b - c
diff --git a/src/calc_test.py b/src/calc_test.py
index 2911910..4820bf3 100644
--- a/src/calc_test.py
+++ b/src/calc_test.py
@@ -16,7 +16,11 @@ def test_add_zeroes(self):
     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')
 
+    def test sub_3arg_zeroes(self):
+        self.assertEqual(sub(0, 0, 0), 0, 'zero minus zero minus zero is zero')
 
 if __name__ == '__main__':
     unittest.main()