Cobalt (Co), copper (Cu), and nickel (Ni) are essential trace elements to life but can cause toxicity when elevated in freshwater. These elements are important commodities to global technological and energy economies. However, Co and Ni have been declared critical minerals for the economic and national security of the United States (U.S.) due to vulnerabilities in their supply chains. Mineral deposits that contain economically recoverable Co also contain either Ni, or Cu and Ni. To sustain economic and national security, previously unexploited domestic Co deposits are likely to be explored and mined. While Cu and Ni are commonly observed in freshwater ecosystems and have aquatic life standards, no standard exist for Co. To address this knowledge gap, we conducted a 30-day mesocosm experiment exposing naïve benthic invertebrate communities to unary, binary, and ternary mixtures of Co, Cu, and Ni. Effect Concentration causing 50% change (EC50) to total mayfly abundance for each unary exposure series were: 11 ug/L for Cu, 18 ug/L for Co, and 38 ug/L for Ni. In biofilms, metal accumulation suggests that inclusion of Cu in mixtures enhanced accumulation of Co and Ni relative to unary exposures, yet Cu accumulation was unaffected by the inclusion of Co or Ni in metal mixture exposures. In filter feeding caddisflies, metal accumulation generally decreased in metal mixture exposures relative to that observed in unary exposures. We employed the response addition model to determine if metal mixtures elicited effects more/less than expected under the assumption of additivity. In all mixtures, invertebrate community responses measured as total mayflies or Bray-Curtis Similarity, suggest that mixtures cause effects at lower concentrations than unary metal exposures caused. While at higher concentrations metal mixtures cause less toxicity than expected. This suggests mixture complexity leads to unexpected toxicity suggesting that Co can be toxic to aquatic life and that mixtures including Cu and Ni do not follow the standard assumption of additivity in all situations.