Raspberry Pi 3
The Raspberry Pi 3 was released in early March 2016, shortly before the release of Ubuntu 14.04 (Xenial) LTS, but too short to have proper support. It is possible to run Xenial's kernel and a full Ubuntu installation, but it requires replacing the firmware, bootloader and some other hacks.
While the BCM2710 is a 64-bit CPU, as of this writing there is no firmware or kernel support for it arm64 mode yet. As such, you will be left with a 32-bit installation.
This procedure assumes a working Ubuntu Xenial Raspberry Pi 2 installation. You will be converting a few things over to work on the RPi3, then doing a hardware swap from an RPi2 to an RPi3.
Be sure you have proper backups! This is not a foolproof or well-tested procedure; if it breaks, you get to keep both pieces.
Xenial upgrade procedure
Install a PPA which contains updated u-boot-rpi, flash-kernel and linux-firmware-raspi2 packages, and upgrade.
add-apt-repository ppa:ubuntu-raspi2/ppa-rpi3 apt-get update apt-get -u dist-upgrade
Install the RPi3 u-boot loader.
wget -O /tmp/mkknlimg https://raw.githubusercontent.com/raspberrypi/tools/master/mkimage/mkknlimg chmod 0755 /tmp/mkknlimg /tmp/mkknlimg --dtok /usr/lib/u-boot/rpi_3_32b/u-boot.bin /boot/firmware/uboot.bin
Remove uboot.env, as the updated flash-kernel package switches to using boot.scr on the RPi3. Run flash-kernel to install the new components.
rm -f /boot/firmware/uboot.env flash-kernel
Check that bcm2710-rpi-3-b.dtb and boot.scr have been created in /boot/firmware by flash-kernel.
Modify /boot/firmware/config.txt with at least the following configs:
enable_uart=1 device_tree_address=0x100 device_tree_end=0x8000 kernel=uboot.bin dtparam=i2c_arm=on dtparam=spi=on
enable_uart is needed by u-boot for all RPi3 modes, even if you have no intention of using a serial console. The device_tree options are needed to explicitly declare the DTB area, as the DTB is slightly larger than 16,128 bytes, and the bootloader otherwise assumes an address space of 0x100 through 0x4000.
Shut down the system.
Swap out the hardware from the RPi2 to the RPi3 and boot.