The brief, which is understood to be worth a significant six-figure sum, spans senior comms advice, press office support, corporate and financial PR, and campaigns. Headland was appointed following a four-round pitching process involving around 15 agencies.
MWW previously held the corporate comms account with Three UK and was among the agencies that pitched for the wider brief, PRWeek understands.
The move to widen the PR brief follows an eventful period at Three UK, which is owned by telecoms giant Hutchinson and has 9.2m customers in Britain.
Yesterday, the group completed its £250m acquisition of UK Broadband Limited, which has 17,000 customers in the UK.
Earlier this year Three UK faced a comms challenge following a data breach that meant some customers were able to view the data of others. It followed a cyber hack in November last year.
In March, Three UK CEO Dave Dyson said the group would have "no option" but to mount a legal challenge to the auction process for the sale of airwaves needed for 5G technology, if smaller players were edged out.
Justin Bass, director of legal & corporate affairs at Three UK, said: "We have an exciting period ahead of us and we recognised that achieving our ambitions requires a different approach. At every stage, Headland has demonstrated strategic and creative thinking, a depth of experience and an ability to deliver a full-service corporate and financial communications offer."
Dan Smith, partner at Headland, said: "Being appointed by Three is a significant moment for Headland. It is one of the most successful challenger brands in the UK, operating in a highly regulated and scrutinised environment. The business is now in a fascinating new stage of growth and we’re delighted they have chosen to work with Headland which, like Three, has been built to challenge the status quo."
A spokesman for MWW said: "We’ve had a great 11 years working with the team at Three, on everything from crisis communications through to high-level corporate issues management, but all good things must come to an end and we wish them the best of luck. We’ve got a strong specialist telecoms team here and we’re expecting to see continued growth in this side of the business in 2017."