--- a/.hgignore
+++ b/.hgignore
@@ -131,16 +131,17 @@ GPATH
 ^testing/talos/bin/
 ^testing/talos/include/
 ^testing/talos/lib/
 ^testing/talos/talos/tests/tp5n.zip
 ^testing/talos/talos/tests/tp5n.tar.gz
 ^testing/talos/talos/tests/tp5n
 ^testing/talos/talos/tests/devtools/damp.manifest.develop
 ^talos-venv
+^py3venv
 
 # Ignore files created when running a reftest.
 ^lextab.py$
 
 # tup database
 ^\.tup
 
 # Ignore Visual Studio Code workspace files.

--- a/testing/mozharness/mozharness/mozilla/testing/talos.py
+++ b/testing/mozharness/mozharness/mozilla/testing/talos.py
@@ -570,19 +570,19 @@ class Talos(TestingMixin, MercurialScrip
         self.buildbot_status(parser.worst_tbpl_status,
                              level=parser.worst_log_level)
 
     def fetch_python3(self):
         manifest_file = os.path.join(
             self.talos_path,
             'talos',
             'mitmproxy',
-            self.config['python3_manifest'][self.platform_name()])
+            self.config.get('python3_manifest')[self.platform_name()])
         output_dir = self.query_abs_dirs()['abs_work_dir']
         # Slowdown: The unzipped Python3 installation gets deleted every time
         self.tooltool_fetch(
             manifest_file,
             output_dir=output_dir,
-            cache=self.config['tooltool_cache']
+            cache=self.config.get('tooltool_cache')
         )
         python3_path = os.path.join(output_dir, 'python3.6', 'python')
         self.run_command([python3_path, '--version'], env=self.query_env())
         return python3_path

--- a/testing/talos/mach_commands.py
+++ b/testing/talos/mach_commands.py
@@ -64,16 +64,20 @@ class TalosRunner(MozbuildObject):
             'default_actions': [
                 'populate-webroot',
                 'create-virtualenv',
                 'setup-mitmproxy',
                 'run-tests',
             ],
             'download_tooltool': True,
             'talos_extra_options': ['--develop'] + self.talos_args,
+            'python3_manifest': {
+                'win32': 'python3.manifest',
+                'win64': 'python3_x64.manifest',
+            }
         }
 
     def make_args(self):
         self.args = {
             'config': {},
             'initial_config_file': self.config_file_path,
         }