What cells would have the CD3 marker on them (other than T-cells)
CD3 is initially expressed in the cytoplasm of pro-thymocytes, the
  stem cells from which T-cells arise in the thymus. The pro-thymocytes
  differentiate into common thymocytes, and then into medullary
  thymocytes, and it is at this latter stage that CD3 antigen begins to
  migrate to the cell membrane. The antigen is found bound to the
  membranes of all mature T cells, and in virtually no other cell type,
  although it does appear to be present in small amounts in Purkinje
  cells. This high specificity, combined with the presence of CD3 at
  all stages of T cell development, makes it a useful
  immunohistochemical marker for T cells in tissue sections. The antigen
  remains present in almost all T-cell lymphomas and leukaemias, and can
  therefore be used to distinguish them from superficially similar
  B-cell and myeloid neoplasms.


Source: http://en.wikipedia.org/wiki/CD3_(immunology)