SL-401 in Combination With Azacitidine or Azacitidine/Venetoclax in Acute Myeloid Leukemia (AML), High-Risk Myelodysplastic Syndrome (MDS) or Blastic Plasmacytoid Dendritic Cell Neoplasm (BPDCN)

Inclusion Criteria: Histologically confirmed diagnosis of acute myeloid leukemia (AML) \[Cohort B\] or myelodysplastic syndrome (MDS) \[Cohort A\] or BPDCN \[Cohort C\] per 2016 WHO criteria CD123 / IL3RA expression on the subject's AML or MDS blasts or BPDCN cells determined locally within 3 months of first protocol treatment Age \>= 18 years with relapsed or refractory AML (hydroxyurea is not considered a prior treatment regimen) \[Cohort B\] OR Age \>= 18 years with treatment-naïve AML who decline intensive induction chemotherapy or who are unfit due to co-morbidity or other factors (see APPENDIX A for unfitness definitions) (hydroxyurea is not considered a prior treatment regimen) \[Cohort B\] OR Age \>= 18 years with MDS and \> 10% myeloblasts in the bone marrow \[Cohort A\] OR Age \>= 18 years with relapsed or refractory BPDCN (hydroxyurea is not considered a prior treatment regimen) \[Cohort C\] Adequate organ function as defined by: Albumin \> 3.2 g/dL (in the absence of receipt of intravenous albumin in the previous 72 hours) Serum creatinine \< 1.5x ULN Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) \< 2.5x ULN Total bilirubin \< 1.5x ULN (if thought to be \> 1.5x ULN due to Gilbert's disease or the patient's AML, must discuss with the PI) Creatine phosphokinase (CPK) \< 2.5x ULN Left ventricular ejection fraction \> institutional lower limit of normal by MUGA scan or echocardiogram within 30 days of first protocol treatment \[Cohort…