This disclosure describes multi-fluid kits for three-dimensional printing, three-dimensional printing kits, and methods of sensing metal ions using three-dimensional printed ion sensors. In one example, a multi-fluid kit for three-dimensional printing can include a fusing agent and an ion-sensing agent. The fusing agent can include water and an electromagnetic radiation absorber. The electromagnetic radiation absorber can absorb radiation energy and convert the radiation energy to heat. The ion-sensing agent can include water and a redox-active inorganic salt.