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"Number Three" is a model of humanoid Cylon, female, blonde, and marked by a religious curiosity—notably surrounding the verboten Final Five—that sets her against the other Cylon models, notably the Number Ones.
The model was first introduced in "Final Cut" as an infiltrator in the Fleet under the alias "D'Anna Biers" posing as a reporter for the Fleet News Service. She appears to be more of a tabloid rumor-and-innuendo format news journalist and less of the conventional Fleet journalist types such as James McManus.
Because of their defiance of the taboo against learning more about the Final Five, the entire line is boxed, and their consciousnesses are stored at the Cylons' central Resurrection Hub (The Ties That Bind).
Natalie's faction in the Cylon Civil War wants to unbox the Number Three who visited the Temple of Five, so that the seven and the five may be united. They eventually forms a fragile alliance with the Colonials to retrieve this Three, and then destroy the Hub (Guess What's Coming to Dinner?). Three is resurrected, and with the destruction of the Hub she becomes the sole Three in existence ("Sine Qua Non", "The Hub").
The first copy of Number Three is introduced in "Final Cut", posing as a documentary filmmaker chronicling the "Gideon massacre" and, later, life aboard Galactica. Her report is transmitted back to Cylon-occuped Caprica, along with information on Sharon Agathon neé Valerii's survival and successful pregnancy.
Her status following the arrival of the Cylons on New Caprica is uncertain but she likely joined the Cylons there as her model would have been exposed by the appearance of other Number Three's.
This Number Three is first introduced in "Downloaded" in her attempt to box Caprica-Six and Sharon "Boomer" Valerii. After the two Cylons convince the others that they made a mistake when they launched their all-out attack on the Colonies, she later oversees the occupation of New Caprica, the recovery of Hera Agathon from Colonial hands, and later leads the Cylons' search for both Earth and the Final Five.
This later search for the Final Five, verboten by a cultural imperative of Cylon society, leads her to the Temple of Five where she receives a vision that identifies the Five. She later dies as a result of this vision, only to be resurrected and summarily boxed, a process overseen by the Number Ones. However, a Cylon Civil War instigated by Natalie (a Number Six) leads to her later unboxing by Cavil and Boomer, with the belief that she can convince the rebels to stop rebelling. She offers her rather violent reply to this (by killing Cavil), and is summarily rescued by Karl Agathon and an Eight, and—with the destruction of the Resurrection Hub—is the last Three in existence and the only Three to ever reach Earth.
A Number Three demonstrates her oratorical skills by giving a speech at the graduation ceremony of the New Caprica Police and is blown up shortly afterwards, along with many of the other attendees, by Tucker "Duck" Clellan, whom she has just congratulated (Occupation).
Later on, two Threes form part of the Cylon counsel in the command center of a baseship, where they first learn of the virus which threatened to infect their ship. After Baltar returns with further information from the baseships, one of the Threes makes the decision to abandon the infected baseship to its fate, angering the Hybrid (Torn).
A Number Three on another baseship repeatedly taunts and tortures Daniel Novacek prior to his seeming escape. Novacek "kills" this copy, which is allegedly diseased, and makes his "escape" to the Fleet. However, this was all part of a Cylon plan for Novacek to take revenge on William Adama, and his exit is observed by another Three, who laughs mockingly (Hero).
Another Number Three refuses the Cylon consensus that the fleet of Heavy Raiders carrying D'Anna Biers and Gaius Baltar to the algae planet must be told to stand down (Eye of Jupiter). Instead, she gives the return order to all the craft except the one with Biers and Baltar on it, reasoning that Admiral Adama would never destroy the Temple of Five and kill his own people over just one ship. This incident makes it clear to the other models that Biers no longer cares about consensus, and that the other Threes are with her (Rapture).
All Number Threes are subsequently boxed (Rapture). Biers herself is revived during the Battle of the Resurrection Hub, and all the other copies are lost with the destruction of the Resurrection Hub (The Hub). Since she is the last, her death marks the extinction of her model.