The first untagged decay-time-integrated amplitude analysis of B s 0  → K S 0 K±π∓ decays is performed using a sample corresponding to 3.0 fb−1 of pp collision data recorded with the LHCb detector during 2011 and 2012. The data are described with an amplitude model that contains contributions from the intermediate resonances K*(892)0,+, K 2 * (1430)0,+ and K 0 * (1430)0,+, and their charge conjugates. Measurements of the branching fractions of the decay modes B s 0 → K*(892)±K∓ and B0s→K∗(---)(892)0K0(---) are in agreement with, and more precise than, previous results. The decays B s 0  → K 0 * (1430)±K∓ and B0s→K∗0(---)(1430)0K0(---) are observed for the first time, each with significance over 10 standard deviations.