I'm still struggling to get Ubuntu 16.04 LTS on my Pi3. Official images don't work. Where did you find the image: ubuntu-minimal-16.04-server-armhf-raspberry-pi.img.xz? Or, have you found a different 16.04 LTS image that works?
I found this link and the ubuntu-16.04-preinstalled-server-armhf+raspi3.img.xz image got properly installed in Raspberry pi 3. One thing to note is that it is a server image so no UI will be there by default.
I took an empty SD card and then use the instructions mentioned in the same page for "Making a microSDHC". After rebooting it started with ubuntu with kernel version 4.4.0-1009-raspi2.