For HepG2 cells, try TrypLE or Accutase. They are more gentle than trypsin and should preserve better the cell surface proteins. Monocytes and T cells are in suspension already.
you may use a scrapper (so called "rubber policeman") which allows to gently put the cells in suspension and maintains a good viability as well as surface antigens. Such a procedure as been frequently used for the propagation of subcultures and keeps a large proportion of the cells alive