There are a few 32-bit boards, but if you grab a random 3D printer, its brain is going to be an 8-bit AVR running something like Marlin or Repetier. It isn’t uncommon to see a Raspberry Pi ...
If you’ve been hanging around 3D printing ... the Raspberry Pi appears to be the platform of choice for most people. At $10 it’s pretty hard to turn down the Pi Zero W as an OctoPrint host ...