diff --git a/src/calc_test.py b/src/calc_test.py
index 2911910..f71d06e 100644
--- a/src/calc_test.py
+++ b/src/calc_test.py
@@ -16,6 +16,8 @@ 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')
 
 
 if __name__ == '__main__':