Sorry to but in but the last post is incorrect RC14 image will run on the Pi3 3 But you have to change the firmware.
As you have a Pi1 one your best option is to create an image using the RC 14 image file write this to an SDcard use a small one in an Micro SD adapter so that you can use it later in your Pi3 the image is only small so will only create a 2GB partition.
put this into the pi one and boot into Risc OS
Now download this File http://www.svrsig.org/software/update4.zip
to update the firmware and the Risc Image(Rom)
Also down load the latest nightly build for the hard Disk here https://www.riscosopen.org/zipfiles/pla ... 1470729053
Next step update the current boot
First unpack the nightly build into a folder on the Pi1 running Risc OS this can then be used to update the current hard disc image
that you are using.
to do this double click on the !Boot folder to open Configuration left click boot from the dialog then left click install to open the merge dialog follow instructions by dragging the new !boot from the folder you unpacked and merge this will update the !boot to the latest version, do not restart yet.
Next unpack update 4 to a folder this has to be copied over your !Boot to update the the firmware and the Risc OS image so that it will work on a Pi Any version.
you can test that it has worked by rebooting your Pi 1 to check it then
you are ready to test it on your Pi 3 don't forget that you will also need to update some more software !Netsurf web browser but you can use !store to do this load it from the apps folder this is because the Pi 3 Is Arm 8 not Arm 6.
If You need any more info just ask.