RecruitingPhase 1Drug trialHypofractionated Radiation in Combination With B7-H3-CAR T Cells for Pediatric Patients With Relapsed/Refractory SarcomasRAD3CAR is a phase I study designed to evaluatethe safety of B7-H3-CAR T cells and lymphodepletion in combination with hypofractionated radiation therapy. Primary objective: \- To evaluate the safety of B7-H3-CAR T cell therapy after priming with hypofractionated radiation therapy (HFRT) and lymphodepleting chemotherapy in patients ≤ 21 years of age with relapsed/refractory B7-H3+ sarcomas. Secondary objectives: * To describe the antitumor activity of B7-H3-CAR T cells in combination with HFRT * To determine if B7-H3-CAR T cells traffic to tumor sites after combination treatment with HFRT