A Study to Evaluate Safety, Pharmacokinetics, and Clinical Activity of Combination of RO6870810 and Venetoclax, With or Without Rituximab, in Participants With Relapsed/Refractory DLBCL and/or High-Grade B-Cell Lymphoma and/or High Grade B-Cell Lymphoma With MYC and/or BCL2 and/or BCL6

Inclusion Criteria * Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2. * Life expectancy \>3 months as per investigator's assessment. * Part 1 and Part 2 Group 1: Participantts with diffuse large B-cell lymphoma (DLBCL) relapsed or refractory to ≥ 1 course of chemotherapy including an anti-CD20 monoclonal antibody, and not eligible for autologous stem cell transplantation (ASCT) (including due to chemorefractory disease). Participants with transformed FL are eligible, provided DLBCL or HGBL-DH/TH histology is biopsy-confirmed prior to study entry and a treatment regimen as described above has been administered. The Sponsor retains the option to limit the number of participants enrolled with transformed FL. Part 2, Group 2: Patients identified with DE-DLBCL (expression MYC ≥40%, BCL2 \> 50%) and or HGBL-DH/TH, relapsed or refractory to \>= 1 course of chemotherapy including an anti-CD20 monoclonal antibody, and not eligible for ASCT (including due to chemorefractory disease). Patients with transformed follicular lymphoma (FL) are eligible, provided DE-DLBCL and/or HGBL-DH/TH histology is biopsy-confirmed prior to study entry and a treatment regimen as described above has been administered. The Sponsor retains the option to limit the number of participants enrolled with transformed FL. * Part 1 and Part 2: Willing to provide the protocol specified tumor biopsy(ies): at screening a fresh biopsy (if no archival biopsy tissue of less than 3 months prior to tre…