I've put together an example of how to make the mini ISO bootable on the pi3.
Download the arm64 iso http://ports.ubuntu.com/dists/bionic/ma ... t/mini.iso
and the device-tree http://ports.ubuntu.com/dists/bionic/ma ... pi-3-b.dtb
Extract and save them to a fat formatted sd card. Make sure you copy the hidden ".disk" folder.
Mount the file /boot/grub/efi.img (on standard Ubuntu 18.04 you can do this by just double clicking on it, but you may have to use a command). Copy the contents to the sd card.
Download the U-boot.bin from opensuse and save to the sd card.
Download the latest pi bootloader files. Create a config.txt file. Save them to the sd card.
If you've done all that you should end up with something like: https://1drv.ms/u/s!AvHY_kl4hMB4f8Fjt7tdOpi9_JM
I've not really used it except to check it boots, but I've noticed the module for the internal sdcard reader is currently missing from the debian-installer. You should be able to install to a USB drive, or once the debian installer has loaded take the sd card out and stick it in a USB reader for the installation.