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