Recent global developments lead companies to include into their strategic plans not only economic sustainability but environmental and social sustainability as well. Companies have been investing in environmental and social sustainability to meet stakeholder demand and/or regulatory demands. Considering this as a market mechanism, we view the sustainability actions of companies as interrelated strategic decisions and propose a Stackelberg game to model the effects of competition for sustainability and sustainability spillovers over the sustainability outcomes of companies. We provide equilibrium solutions for the one leader, two followers game over different intervals of competition levels and spillover rates. Using a numerical example, we observe how the sustainability investments and net benefits change as competition levels and spillover rates change and identify the competition-spillover regions, where each player invests the most and has the advantage in terms of benefit. We discuss implications for both the companies and the policy makers.