The confirmation comes weeks after reports that Claydon Heeley was expected to take the business, following the elimination of the three other agencies involved in the pitch -- Catchpole & Friends, Hicklin Slade and Lida.
Claydon Heeley partnered with digital specialist Holler because of conflict issues from its sister network Agency Republic, which handles online advertising for O2 and was unable to pitch.
The account covers 3's communications to existing customers across all media, with an emphasis on the digital channels familiar to its young user base.
Gary Pepler, director of brand and marketing communications at 3, said: "Claydon Heeley's insight into our business needs was second to none."
Mike Welsh, managing director of Claydon Heeley, said: "This appointment is a fabulous vote of confidence in our new positioning as a player in the digital world."
3 has previously used a portfolio of different agencies for its customer communications activity.
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