Brief Summary: This phase I study studies the side effects and best dose of venetoclax and lenalidomide when given together with obinutuzumab in treating patients with B-cell non-Hodgkin lymphoma that has returned after a period of improvement or not responding to treatment. Monoclonal antibodies, such as obinutuzumab, may interfere with the ability of cancer cells to grow and spread. Venetoclax may stop the growth of cancer cells by blocking some of the enzymes needed for cell growth. Drugs used in chemotherapy, such as lenalidomide, work in different ways to stop the growth of cancer cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Giving obinutuzumab, venetoclax, and lenalidomide may work better in treating patients with B-cell non-Hodgkin lymphoma.

Detailed Description: PRIMARY OBJECTIVES: I. To determine the dose limiting toxicity (DLT) and the recommended phase 2 dose (RP2D), which is typically the maximum tolerated dose (MTD), of the combination of obinutuzumab, venetoclax, and lenalidomide in patients with relapsed or refractory B-cell non-Hodgkin lymphoma (NHL). SECONDARY OBJECTIVES: I. To estimate the overall objective response rate to the combination of obinutuzumab, venetoclax, and lenalidomide in patients with relapsed or refractory B-cell NHL. II. To estimate the duration of response and 2 year progression-free survival associated with obinutuzumab, venetoclax, and lenalidomide treatment in patients with relapsed and refractory B-cell NHL. III. To define the qualitative and quantitative toxicities of the combination of obinutuzumab, venetoclax, and lenalidomide in patients with relapsed or refractory B-cell NHL. OUTLINE: This is a dose-escalation study of venetoclax and lenalidomide. Patients receive lenalidomide orally (PO) on days 1-21 and venetoclax PO on days 1-28. Patients also receive obinutuzumab intravenously (IV) on days 1, 8, and 15 of course 1 and on day 1 of courses 2-6. Treatment repeats every 28 days for up to 12 courses in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed up at 4 weeks, every 3 months for 2 years, then every 6 months.