This phase 3 study is recruiting patients who are at risk of graft-versus-host disease (GVHD) after a bone marrow transplant. This study will measure the safety and effectiveness of a prevention treatment combination called Tacrolimus/Methotrexate/Ruxolitinib compared to Standard of Care (SOC), Post-Transplant Cyclophosphamide/Tacrolimus/Mycophenolate Mofetil. Ruxolitinib (Rux) is an approved treatment for GVHD. This study is divided into two parts. The first part, called the run-in phase, will investigate the best dose of Ruxolitinib. The second part of the study will compare the SOC combination therapy with the investigational combination therapy (which will include Ruxolitinib). Participants will be randomly assigned to one of the two groups (like flipping a coin). The study will enroll up to 572 patients nationwide and 5 at MUSC. The participants can expect to be involved in the study for at least 24 months. The main risk is that medical treatments often cause side effects. The most common side effects expected from the investigational combination therapy are high cholesterol, increased liver enzymes, low platelet levels, and low red blood cell counts. There is no direct benefit for them in participating in this study.