The £38 million business was the agency's biggest- spending client in 2003 although, for this year, TBWA claims that Abbey and Masterfoods are its most profitable pieces of business.
The 3 account was shifted into WCRS last week. The agency had been working on 3's strategy since the end of last year.
TBWA held the 3 account since 2002, repitching for the business last summer against WCRS, Delaney Lund Knox Warren & Partners, Bartle Bogle Hegarty and M&C Saatchi.
It handled the launch of 3 into the UK market in 2003, introducing the strapline, "You to the power of 3" in national TV, outdoor and press ads.
Its most recent work for the client was a branding campaign that used the Brookside actress Anna Friel. The use of Friel was adopted by WCRS in ads it created to promote 3's range of Prepay services earlier this year.
TBWA refused to comment on the redundancies.