This study will be evaluating the safety and tolerability of CFI-400945 in subjects with Acute Myeloid Leukemia, Myelodysplastic Syndrome or Chronic Myelomonocytic Leukemia. The study is designed to build on encouraging data from another study and to obtain further safety, efficacy, pharmacokinetics (PK) and pharmacodynamics (PD) data of CFI-400945.