This disclosure relates in general to three dimensional (“3D”) printers having a configuration that prepares a three-dimensional object by using a feedstock comprising a metal or a polymer compound and a carbon coating formed on a surface of the compound. This disclosure also relates to such feedstocks and their preparation methods. This disclosure further relates to 3D composite objects prepared by using such printers and feedstocks. This disclosure also relates to carbon containing photocurable formulations and methods for their preparation. This disclosure further relates to electrically conducting 3D polymer composites prepared by using such carbon containing photocurable formulations.