The Havas-owned agency won the brief after a three-way pitch against Hurrell and Dawson and Glue. It is understood that Glue will still pitch for certain projects for the mobile brand in the future.
John Penberthy-Smith, marketing director of 3 UK, said: "We were looking for three things; great planning, great creative and most importantly, great chemistry.
"Euro RSCG had them all. We wanted an agency that can work round the kitchen table and they were the best fit. 3 is a challenger brand and we wanted to work with a team that is using its experience to challenge the norms in the world of communications, as we set out to do in the world of telecoms and media."
Mark Cadman, chief executive of Euro RSCG London, said: "We're delighted to be working with 3. They ran a thorough and detailed process, during which we got to know each other well and we're looking forward to tackling the challenges that lie ahead."
The handover from WCRS, 3's existing agency, will take place during the summer.