Jamieson, Fausset and Brown on Isaiah 16:14

14. three years ... hireling--Just as a hireling has his fixed term of engagement, which neither he nor his master will allow to be added to or to be taken from, so the limit within which Moab is to fall is unalterably fixed (Is 21:16). Fulfilled about the time when the Assyrians led Israel into captivity. The ruins of Elealeh, Heshbon, Medeba, Dibon, &c., still exist to confirm the inspiration of Scripture. The accurate particularity of specification of the places three thousand years ago, confirmed by modern research, is a strong testimony to the truth of prophecy.