A negative E value means that the oxidized species is more stable than the reduced species, the metals will easily lose their electrons and get oxidized. Here, Mn²⁺(3d⁵) and Zn²⁺(3d¹⁰) have half-filled and fully filled d orbitals which gives them stability and therefore prefer to stay that way and not get reduced. As for Ni²⁺(3d⁸), it has very high negative hydration enthalpy which gets balanced by first and second ionization enthalpy.