The present disclosure describes three-dimensional printing kits, three-dimensional printing systems, and methods of making three-dimensional printed objects. In one example, a three-dimensional printing kit can include a build material and a binding agent. The build material can include particles of copper or a copper alloy. The binding agent can include water, copper (II) nitrate or a hydrate thereof, and a reaction inhibition additive. The additive can be a copper oxide etchant, a water-soluble phosphate-containing compound, or a combination thereof.