You can first sand the item with very high grit paper. Then use buffing wheels, for example on a small Dremel tool, with polishing paste. There are ultrasonic cleaners available to remove polishing residue.

Be aware that in some 3D printing processes, the outer layer is added by electroplating. In that case, be very careful with finishing as the outer layer is only a few microns thick. This makes many 3D printing processes not suitable for creating durable jewelry.