From what I see missing any line of the type
kernel=vmlinuz-XXXXX
and therefore the operating system is not realtime.
If you want an image that definitely works on Raspberry Pi 3 you can use the one you find here:
forum.linuxcnc.org/9-installing-linuxcnc...3b-pi?start=0#260915
you are right, the system isn't running the preempt-rt kernel.
proper uname (except for aarch64) should be
Linux raspberrypi 5.15.65-rt49-v8+ #1 SMP PREEMPT_RT Tue Sep 6 18:12:35 BST 2022 aarch64 GNU/Linux
