This open-label, randomized, multicenter, triple-arm Phase Ib/II study is designed to assess the efficacy, safety, tolerability, and pharmacokinetics of cobimetinib administered as a single agent (Arm A), cobimetinib plus venetoclax (Arm B), and cobimetinib plus venetoclax plus atezolizumab (Arm C) in participants with relapsed and refractory multiple myeloma. Two successive cohorts will evaluate the safety of cobimetinib plus venetoclax and that of cobimetinib plus venetoclax plus atezolizumab in the selected population during the safety run-in phase of the study. Once the dose levels have demonstrated acceptable safety during this phase, randomization will begin for all treatment arms (Arms A, B, and C).