diff --git a/contrib/automation/hgautomation/cli.py b/contrib/automation/hgautomation/cli.py
--- a/contrib/automation/hgautomation/cli.py
+++ b/contrib/automation/hgautomation/cli.py
@@ -480,7 +480,7 @@
             'pypy3.5',
             'pypy3.6',
         },
-        default='system2',
+        default='system3',
     )
     sp.add_argument(
         'test_flags',