This phase II trial tests the safety, side effects and how well giving tafasitamab with acalabrutinib and venetoclax works for the treatment of chronic lymphocytic leukemia (CLL)/small cell lymphoma (SLL). A monoclonal antibody is a type of protein that can bind to certain targets in the body, such as molecules that cause the body to make an immune response (antigens). Tafasitamab is a monoclonal antibody that binds to CD19 antigen which is found on the surface of most B cells (a type of white blood cell) and some lymphoma cells. This may help the immune system kill cancer cells. Acalabrutinib is in a class of medications called kinase inhibitors. It blocks a protein called BTK, which is present on B-cell (a type of white blood cells) cancers such as mantle cell lymphoma at abnormal levels. This may help keep cancer cells from growing and spreading. Venetoclax is in a class of medications called B-cell lymphoma-2 (BCL-2) inhibitors. It may stop the growth of cancer cells by blocking Bcl-2, a protein needed for cancer cell survival. Giving tafasitamab with acalabrutinib and venetoclax may be safe and effective for treating patients with CLL/SLL.