The purpose of this study is to learn about the effects of three study medicines (encorafenib, cetuximab, and pembrolizumab) given together for the treatment of colorectal cancer that: * is metastatic (spread to other parts of the body); * has the condition of genetic hypermutability (tendency to mutation) or impaired DNA mismatch repair (MMR) * has a certain type of abnormal gene called "BRAF" and; * has not received prior treatment. All participants in this study will receive pembrolizumab at the study clinic as an intravenous (IV) infusion (given directly into a vein) at the study clinic. In addition, half of the participants will take encorafenib by mouth at home every day and cetuximab by IV infusion at the study clinic. The study team will monitor how each participant is doing with the study treatment during regular visits at the study clinic.