As their name suggests, III Isaura and its twin legion II Isaura were guarding the Isauria territory at the time of the Notitia Dignitatum, to defend it from the incursions of the mountain peoples.
The legions III Isaura and II Isaura, as indicated by their name, were stationed in the Isauria area during the Notitia Dignitatum, tasked with defending it against the incursions of the mountain-dwelling peoples.