The news comes as a blow to TBWA, which successfully repitched for the mobile phone network in a whirlwind two-week process last summer. It was originally appointed in 2002.
WCRS, which was involved in the pitch along with Bartle Bogle Hegarty, Delaney Lund Knox Warren & Partners and M&C Saatchi, began working on the account in a strategic capacity at the end of last year.
In February this year, the agency persuaded the company to run its ads promoting 3's range of Prepay services. The commercials featured the former Brookside actress Anna Friel, who was chosen by TBWA to front the main branding campaign.
WCRS has considerable experience in the mobile telecoms arena, having launched Orange in the UK in 1994. It has also handled the UK Vodafone account.
The agency has close ties with senior management at 3, many of whom worked with WCRS at Orange. The agency was in talks with 3 before the advertising review in June last year.
The account win is a boost for WCRS, which has just completed a management buyout from its parent company, Havas.
Andrew McGuinness, the chief executive of TBWA, said: "It's been a tough ride over the past three years. We are clearly disappointed."
MindShare is the incumbent on 3's media planning and buying account, which is not thought to be affected by the creative appointment.
Neither WCRS nor 3 were not available to comment on the appointment as Campaign went to press.