Canonical provided instructions for how to install Ubuntu from generic ARM64 ISOs on the Pi 3. Since the tutorial uses the minimal installer, you can select components such as "Lubuntu desktop" and "Xubuntu desktop" and install those.
https://wiki.ubuntu.com/ARM/RaspberryPi ... ISO_images
Benefits of installing Ubuntu from the minimal installer ISO:
[*]You get 64-bit versions of various Ubuntu 18.04 flavours running on the Pi.
[*]Demonstrates the Pi 3's ability to run ARM64 software.
[*]Apparently, it works on the Pi 3B+ as well, whereas the Ubuntu MATE image available from ubuntu-mate.org does not.
[*]The Ubuntu Pi Flavour Maker flavours are stuck at 16.04, whereas the minimal installer lets you install 18.04 and 18.10.
Cons of installing Ubuntu from the minimal installer ISO:
[*]The ISOs were not designed for use on the Pi.
[*]It's not as simple as flashing the ISO to a USB stick and plugging it into your Pi. Some modding is required so that the Pi can boot from the ISO straight from the SD card.
[*]You need to use Ethernet to install since the installer does not allow wireless installation. Also by default, Ubuntu is not compatible with the Pi 3's built in WiFi, so you'll have to install some drivers.
Please note that the following Ubuntu flavours are compatible:
[*]Kubuntu (with desktop effects turned off)
The "full" Ubuntu with the GNOME 3 desktop doesn't work, probably because of GPU incompatibility. Ubuntu Budgie MIGHT work, but I haven't tried it yet.