The purpose of this study is to evaluate a study drug called CFI-400945, either by itself or in combination with one of two chemotherapy drugs, known as azacitidine or decitabine. The study aims to determine how safe this drug is at different doses, with food, and in combination with azacitidine or decitabine, and to measure how well each combination works for acute myeloid leukemia (AML), myelodysplastic syndrome (MDS), or chronic myelomonocytic leukemia (CMML).