Study to Investigate the Safety and Clinical Activity of GSK3326595 and Other Agents to Treat Myelodysplastic Syndrome (MDS) and Acute Myeloid Leukemia (AML)

Inclusion Criteria:

* Males and females greater than or equal to (\>=)18 years of age (at the time consent is obtained).
* Diagnosis of MDS, CMML or AML
* Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1 or 2
* Adequate organ function
* A woman of childbearing potential (WOCBP) must have a negative highly sensitive serum pregnancy test within 7 days before the first dose of study intervention.

Exclusion Criteria:

* History of, or known, central nervous system (CNS) involvement
* Prior solid organ transplantation
* Known allergies, hypersensitivity, or intolerance to GSK3326595 or 5-Azacitidine or its excipient
* Prior therapy with any Protein arginine methyl transferase 5 (PRMT5) inhibitor
* History of a second malignancy, excluding non-melanoma skin cell cancer, within the last three years
* Active severe or uncontrolled infection
* History of optic nerve neuropathy or neuritis.
* History of sensitivity to any of the study medications, or components thereof, or a history of drug or other allergy that, in the opinion of the investigator or Medical Monitor, contraindicates their participation.