RISC OS runs very well on the Raspberry Pi 3B.
Unfortunately there are a few utilities that have not yet been updated for the ARMv8 CPU. That I have found so far these include:
There are of course more that do not yet work.
And for the RPi 3B you need to have at least version 3.5 of Netsurf. This can be a problem, as the RPi SD image is set up to automaticaly start an older version of Netsurf on the first boot to display a getting started page, and this locks the system. Though you can bypass this by pressing escape as soon as the boot sequence begins (to stop it from running the boot sequence), then install a newer version of Netsurf (which you will have to have downloaded with a different system), and reboot.
Also you need a newer ROM than what comes with the RC14 SD Card image, this can be replaced easily though. It is also recommended that you update the !boot sequence.
Though as soon as !PDF gets updated I will be using the RPi 3B as my main RISC OS system, as it is the only thing that I need on a regular bases that does not yet run on the RPi 3B.