Inclusion Criteria:

* Age ≥18 years at time of enrollment
* Diagnosis of one of the following by World Health Organization (WHO) International Consensus Classification (ICC) criteria as determined by local assessment:

* Relapsed/ refractory acute myeloid leukemia (R/R AML) as defined by ≥5% marrow blasts or unequivocal, measurable extramedullary disease
* High Risk Myelodysplastic Syndrome (HR-MDS) (high/very high risk MDS by Revised International Prognostic Scoring System (IPSS-R) or Molecular International Prognostic Scoring System (IPSS-M)
* high-risk accelerated-phase myeloproliferative neoplasm (HR/AP-MPN) defined by ≥10% blasts in blood or bone marrow
* Eastern Cooperative Oncology Group (ECOG) Performance status 0-3
* White blood cell (WBC) count ≤25 × 109/Liter (L) (cytoreduction with hydroxyurea or steroids is allowed to achieve this)
* Aspartate Aminotransferase (AST)/ Alanine Aminotransferase (ALT) ≤3 × upper limit of normal (ULN) (≤5 × ULN if due to leukemic involvement)
* Total bilirubin ≤2 × ULN (unless the elevation is due to Gilbert's or hemolysis)
* Creatinine clearance ≥ 30 milliliters / minute (mL/min)
* Women of child-bearing potential must not be pregnant or breastfeeding and must have a negative pregnancy test at screening. Women of non-childbearing potential are those who have had a hysterectomy or bilateral oophorectomy, or who have completed menopause (no menses for at least one year and age ≥65 or follicle-stimulating hormone levels in the menopausal range).
* Subjects and their partners with reproductive potential must agree to use effective contraceptive measures during the study and for 3 months after the last dose of study treatment. Effective contraception includes methods such as oral contraceptives or double-barrier method.

Exclusion Criteria:

* Prior use of hypomethylating agent and venetoclax in combination (Note, use of hypomethylating agent and/or venetoclax separately in alternative combinations with other drugs is allowed)
* Inability to tolerate oral therapies, or medical co-morbidities that significantly impact parenteral absorption
* Acute promyelocytic leukemia myeloproliferative neoplasm (MPN) with the Philadelphia chromosome translocation (BCR:ABL) translocation
* Clinically significant cardiovascular disease as defined by unstable angina
* New York Heart Association class III/IV congestive heart failure
* Treatment with any investigational drug or therapy within 2 weeks of study treatment or 5 half-lives before the first dose of study treatment, whichever is shorter
* Known hypersensitivity to azacitidine, venetoclax, decitabine or cedazuridine
* Cytotoxic chemotherapy or prior azacitidine or decitabine within 2 weeks of first dose of study treatment
* Concurrent use of AML/MDS/MPN therapies including lenalidomide, erythropoietin, luspatercept, cytotoxic chemotherapies, targeted agents, etc Note: hydroxyurea is allowed in Cycle 1 if necessary for cytoreduction and/or cytarabine not exceeding a maximum dose of 1 gram per meter squared (g/m2) in Cycle 1 is also allowed for cytoreduction
* Uncontrolled intercurrent illness or infection (those with controlled HIV, hepatitis, or other chronic infections are eligible)
* Untreated central nervous system disease
* Pregnancy or breastfeeding
* Other active malignancy requiring systemic therapy during duration of trial or otherwise would confound endpoints (eg, second malignancy present where survival is expected to be less than 6 months)