I have a "RISCOS Pi" image from the RISCOS webpage as linked on the official Raspberry Pi download page and the RISCOS official download page. I've downloaded the newest version as of this writing, and it boots fine on a Raspberry Pi 1B (512 MB RAM).
But the very same image, even on the same SD card (a 64 GB Transcend microSDXC Class 10 UHS-1) doesn't boot on the Raspberry Pi 3. I only get a square with gradients between (clockwise, starting from top left) red via yellow and cyan to blue with the same square much smaller in the top right corner of the screen again.
The used screen is an AOC i2269VWM connected via HDMI.
Any idea what could have gone wrong? Is it possible that the image itself isn't suitable for the Raspberry Pi 3 at all due to its age? It has been released about one year before the Raspberry Pi 3 was released.
Unfortunately I currently don't have a Raspberry Pi 2 to check if its a Pi-3-only issue or if the image only works on Raspberry Pi Ones at all.