Observations seemingly at odds with the Standard Model of particle physics, including dark matter and the light mass of the Higgs boson, motivate searches at the LHC for new particles predicted by Electroweak Supersymmetry. New results from the ATLAS Collaboration are presented targeting mass resonances decaying into three-lepton final states. The High Luminosity upgrade of the LHC will greatly increase the potential to produce and observe rare new physics but presents unique challenges from a busy, high-radiation environment. Recent developments are highlighted on the design and testing of the upgraded readout electronics for the ATLAS silicon strip tracker.