This is a clinical trial to evaluate the feasibility and safety of giving tazemetostat followed by standard of care CAR T cell infusion in previously treated diffuse large b-cell lymphoma (DLBCL), follicular lymphoma (FL), and mantle cell lymphoma (MCL). The investigators hypothesis is that this combination has the potential to significantly improve the ability of CART cells to recognize and kill lymphoma cells without a significant impact on safety. Participants will receive the tazemetostat pills before and after receiving their CAR T cell therapy, for up to 12 months after CAR T cell administration. Patients will be followed for up to 5 years.