The Webmin utility is helpful when running headless.  It is an excellent safety net when other services, such as ssh or xrdp are unavailable.  Installing it on the Raspberry Pi 3 under Ubuntu Mate is done using these commands from a terminal window.

sudo apt-get install perl libnet-ssleay-perl openssl libauthen-pam-perl
  --->  libpam-runtime libio-pty-perl apt-show-versions python
wget http://prdownloads.sourceforge.net/webadmin/webmin_1.801_all.deb
sudo dpkg --install webmin_1.801_all.deb

Please note, the code above refers to the version available during this writing.  Please check the developers website for the most recent release.  You can find it here, http://www.webmin.com/download.html

Once installed, the webmin page can be accessed from a web browser, <ip of pi>:10000

The real benefit of webmin is the software updates link it offers on the main page.  It’s a friendly reminder that patching is important and shouldn’t be ignored.