OSGeo4W shell with python3

There's no need to edit batch files (and break python 2.7 in the process) or set environment variables manually.

Just run py3_env which sets the shell up for Python 3 automatically, then run python3.

From a OSGeo4W shell with just python 2 & 3 installed:

C:\> o-help
                   -={ OSGeo4W Shell Commands }=-

       dllupdate           pip3                sqlite3
       easy_install3       python              textreplace
       nircmd              python3             vcredist_2015_x64
       nircmdc             pythonw             xxmklink
       osgeo4w-setup       pythonw3


       make-bat-for-py   py3_env  <<<=== This is the command you want
       o-help            setup-test
       o4w_env           setup



C:\>python3
Fatal Python error: Py_Initialize: unable to load the file system codec
ModuleNotFoundError: No module named 'encodings'

Current thread 0x000013e8 (most recent call first):
^C

C:\>py3_env
C:\>python3

Python 3.6.0 (v3.6.0:41df79263a11, Dec 23 2016, 08:06:12) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>

There is a not well documented command build into OSGeo4W Shell which sets the shell up to python3 as Luke mentions

py3_env

Basically it sets your PYTHONHOME and the correct PATH. Then you can call Python3 with python3.

C:\>py3_env
C:\>SET PYTHONPATH=
C:\>SET PYTHONHOME=C:\OSGEO4~1\apps\Python36
C:\>PATH C:\OSGEO4~1\apps\Python36;C:\OSGEO4~1\apps\Python36\Scripts;C:\OSGEO4~1\apps\Python27\Scripts;C:\OSGEO4~1\bin;C:\Windows\system32;C:\Windows;C:\Windows\WBem
C:\>python3
Python 3.6.0 (v3.6.0:41df79263a11, Dec 23 2016, 08:06:12) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>

---

Update with auslander's comment:

To use Python3 with the OSGeo4W shell that comes with QGIS 2 you have to change the file {path you installed qgis}\etc\ini\python-core.bat:

SET PYTHONHOME=%OSGEO4W_ROOT%\apps\Python36
PATH %OSGEO4W_ROOT%\apps\Python36\Scripts;%PATH%

If you want to make the change permanent you can modify the batch file OSGeo4W.bat located in your QGIS (or OSGEO) directory and just add:

call "py3_env"

below the line calling

call "%~dp0\bin\o4w_env.bat"