Brief Summary: 3CAR is being done to investigate an immunotherapy for patients with solid tumors. It is a Phase I clinical trial evaluating the use of autologous T cells genetically engineered to express B7-H3-CARs for patients ≤ 21 years old, with relapsed/refractory B7-H3+ solid tumors. This study will evaluate the safety and maximum tolerated dose of B7-H3-CAR T cells.The purpose of this study is to find the maximum (highest) dose of B7-H3-CAR T cells that are safe to give to patients with B7-H3-positive solid tumors. Primary objective To determine the safety of one intravenous infusion of autologous, B7-H3-CAR T cells in patients (≤ 21 years) with recurrent/refractory B7-H3+ solid tumors after lymphodepleting chemotherapy Secondary objective To evaluate the antitumor activity of B7-H3-CAR T cells Exploratory objectives * To evaluate the tumor environment after treatment with B7-H3-CAR T cells * To assess the immunophenotype, clonal structure and endogenous repertoire of B7-H3-CAR T cells and unmodified T cells * To characterize the cytokine profile in the peripheral blood after treatment with B7-H3-CAR T cells

Detailed Description: Treatment will include a single infusion of B7-H3-CAR T cells after lymphodepleting chemotherapy, with dosing based on the number of CAR+ T cells and patient weight. The study will evaluate the safety and maximum tolerated dose (MTD) of B7-H3-CAR T cells, using a standard 3+3 study design and a 6-week evaluation period. The total study duration will be 1 year, at which point patients will enroll on our existing institutional long-term follow-up protocol.