Acropolis
From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
Though not specifically seen on screen, the battlestar Acropolis is mentioned when the Cylons attack the fleet at Cimtar (Saga of a Star World).
It's likely Pacifica and Triton are the other unnamed battlestars seen with Atlantia and Galactica at Cimtar.