As the complex gives two moles of AgCl precipitate with AgNO3 solution, so the complex must have two ionisable Cl atoms. Hence, the probable complex, which gives three mole ions may be [Co(NH3)5(NO2)]Cl2.
[Co(NH3)5NO2]Cl2→[Co(NH3)5NO2]2++2Cl−
one mole complex→3 mole ions