Epco, Zanu, Ritux for R/R FL or MZL

The purpose of this study is to determine how effective and safe the combination of epcoritamab, zanubrutinib, and rituximab is in treating participants with relapse or refractory Follicular Lymphoma (FL) or marginal zone lymphoma (MZL). * The names of the study drugs involved in this research study are: * Epcoritamab (a type of antibody) * Zanubrutinib (a type of Bruton tyrosine kinase inhibitor) * Rituximab (a type of monoclonal antibody)

Inclusion Criteria: * Histologically confirmed diagnosis of CD20+ FL (grade 1-3A) or CD20+ MZL (any subtype) (at time of trial entry) with review of the diagnostic pathology specimen at one of the participating institutions. Patients with current histologic transformation are excluded. * Receipt of at least one prior line of therapy for FL or MZL (with prior treatment including a CD20 monoclonal antibody). * Measurable disease, defined as ≥1 measurable nodal lesion (long axis \>1.5 cm or short axis \>1.0 cm), or ≥1 measurable extra-nodal lesion (long axis \>1.0 cm), or spleen \>13 cm on PET, CT, or magnetic resonance imaging (MRI). For patients with FL, disease should be FDG-avid based on PET. FDG-avid disease is NOT a requirement for patients with MZL. * Meets at least one criterion to begin treatment based on the modified GELF (Groupe d'Etude des Lymphomes Folliculaires) criteria: * Symptomatic adenopathy * Organ function impairment due to disease involvement, including cytopenias due to marrow involvement (WBC \3 nodal sites of involvement \>3 cm * Local compressive symptoms or imminent risk thereof * Splenomegaly (craniocaudal diameter \> 16cm on CT imaging) * Clinically significant pleural or peritoneal effusion * Leukemic phase (\>5x109/L circulating malignant cells) * Rapid generalized disease progression * Renal infiltration * Bone lesions * Eastern Cooperative Oncology Group (ECOG) performance status of 0-2. (Appendix A) * Age ≥18 years. * Adequate hematologic and organ function: 1. Absolute neutrophil count \> 1.0x109/L unless due to marrow involvement by lymphoma in which case ANC must be \>0.5x109/L 2. Platelets \> 75 x109/L, unless due to marrow involvement by lymphoma, in which case platelets must be \>50 x109/L 3. Estimated CrCl (based on Cockcroft Gault or MDRD) ≥ 45ml/min or ≥ 45ml/min/1.73m2 4. Total bilirubin \< 1.5 X ULN, unless Gilbert syndrome, in which case direct bilirubin must be \< 1.5 x ULN 5. AST/ALT \< 2.5 X ULN, unless documented liver involvement by lymphoma, in which case AST/ALT must be \