Encorafenib, Cetuximab, and Nivolumab in Treating Patients With Microsatellite Stable, BRAFV600E Mutated Unresectable or Metastatic Colorectal Cancer

Inclusion Criteria: * Provision of signed Informed Consent prior to any screening procedures being performed. A translator may be used for patients who do not speak or read English. * Age ≥ 18 years at the time of informed consent. * Histologically (or cytologically) confirmed diagnosis of adenocarcinoma of the colon or rectum, with clinical confirmation of unresectable and/or metastatic disease that is measurable according to RECIST1.1 criteria. * Confirmation of BRAFV600E tumor as detected by a CLIA-certified laboratory. * Confirmation of MSS status in a CLIA-certified laboratory. * Cohort A only: Prior treatment with at least one, but no more than two, systemic chemotherapy regimen(s) for mCRC, or recurrence/progression with development of unresectable or metastatic disease within 6 months of adjuvant chemotherapy for resected colorectal cancer. * Cohort B only: prior treatment with a BRAF, anti-EGFR, MEK, or ERK targeted therapy for treatment of colorectal cancer * ECOG performance status \< 1 * Adequate bone marrow, organ function and laboratory parameters: * Absolute neutrophil count (ANC) ≥ 1.0 x 109/L, * Hemoglobin (Hgb) ≥ 9 g/dL with or without transfusions, * Platelets (PLT) ≥ 100 x 109/L without transfusions, * AST and/or ALT ≤ 2.5 × upper limit of normal (ULN). If liver metastases are present, then it is acceptable for AST level ≤ 5.0 × ULN, and an ALT level ≤ 5.0 × ULN. * Total bilirubin ≤ 1.5 × ULN and \< 2 mg/dL Note: Patients who have a total bilirubin level …