Update: False alarm. The one that came included with my PowerCore+ 26800 PD is also 3ft. Sorry! Again, you my try reaching out to their support team, but at this point, I doubt that will do much to help. Anker apologizes for the inconvenience they have caused you! (and everyone else looking for a shorter USB-C cable).
I don’t think the USB-C cable I received with my PowerCore+ 26800 PD was quite 3ft, but I will try to confirm that when I have access to it later tonight. If what I think is true, you may be able to order one by contacting support@anker.com but that is far from a guarantee.