Although for Cu, Ag, & Au, the oxidation states of +1,+2,+3 are all known; except for Au +2, they are nor of equal stability with each elementThe difference between the first ionisation potential and the second ionisation potential increases in the order Au < Cu < Ag, and the common oxidation states of the metals are Au2+, Cu2+ and Ag+1. Hence Ag3+ will be less stable and hence unreactive towards halogens.