Not sure if you would agree to call them major distros but at least Fedora 23 (Nov, 2015) and Ubuntu 18.04 LTS (Apr, 2018) ships with Python 3 in their base.

> No major Linux distro besides arch ships with 3 as it's base either.

To add one more entry to sslalready's list:

Don't know if you consider Manjaro just an Arch derivative, or not-major (despite being the most popular distro on Distrowatch), but Manjaro also ships with Python3 (to be more exact - Python 3.7) as a base.

    $ /usr/bin/python
    Python 3.7.0 (default, Jul 15 2018, 10:44:58) 
    [GCC 8.1.1 20180531] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>>