Pembrolizumab and Combination Chemotherapy in Treating Patients With Previously Untreated Diffuse Large B-cell Lymphoma or Grade 3b Follicular Lymphoma

Inclusion Criteria: * Previously untreated diffuse large B-cell lymphoma or grade 3B follicular lymphoma (of any stage); subjects must be planned to receive full course (6 cycles) of RCHOP chemoimmunotherapy as per clinical standard of care; patients may have de novo DLBCL, and /or any of the following: * Composite lymphomas, which include both diffuse DLBCL and another histology (most commonly follicular lymphoma) in the same lymph node * Transformed lymphoma with DLBCL histology, as long as the patient has not received prior therapy for lymphoma * Discordant presentations, such as DLBCL in a lymph node and low-grade lymphoma such as follicular lymphoma in the bone marrow * Be willing and able to provide written informed consent/assent for the trial * Have measurable nodal disease, including at least 1 disease site measuring 1.5 cm in longest dimension on computed tomography (CT) or fludeoxyglucose (FDG)-positron emission tomography (PET) * Have a performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) performance scale (PS) * Absolute neutrophil count (ANC) \>= 1,500/mcL except in cases of marrow infiltration by lymphoma * Platelets \>= 100,000/mcL except in cases of marrow infiltration by lymphoma * Hemoglobin \>= 9 g/dL or \>= 5.6 mmol/L except in cases of marrow infiltration by lymphoma * Serum creatinine =\< 1.5 X upper limit of normal (ULN) OR measured or calculated creatinine clearance \>= 60 ml/min for subject with creatinine levels \> 1…