This is a first-in-human, single-arm, open-label study evaluating the safety, tolerability, and preliminary efficacy of ALLO-329 in adults with autoimmune diseases: systemic lupus erythematosus (SLE) with and without renal involvement, idiopathic inflammatory myopathy (IIM), and systemic sclerosis (SSc).The purpose of this trial is to evaluate the safety and tolerability of ALLO-329, an allogeneic anti-CD19, anti-CD70 dual chimeric antigen receptor (CAR) T cell therapy, in adults with autoimmune disorders, provide initial evidence of biological activity and clinical response to the treatment and determine the recommended Phase 2 regimen (RP2R).