It seems to support Python 3 (3.5.4): https://cloud.google.com/appengine/docs/flexible/python/runt...
Support exists at least since August 2016: https://cloudplatform.googleblog.com/2016/08/python-3-on-Goo...
Support exists at least since August 2016: https://cloudplatform.googleblog.com/2016/08/python-3-on-Goo...