PRIMARY OBJECTIVES: I. To assess the feasibility in terms of toxicity and safety of HT in combination with Fludarabine (fludarabine phosphate) and Melphalan as a preparative regimen for allogeneic stem cell transplantation in patients with Hematological Malignancies: acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and myelodysplastic syndromes (MDS).