The company has shortlisted agencies for the work after seven years with the incumbent, which declined to repitch.
Georgie Carter, head of consumer PR at Three, said: "3 Monkeys has helped us tell our story with some great campaigns. After seven years working together we’ve decided it’s time for a review. We have a shortlist in place and will be selecting from it over the next few weeks."
News of the process, which is thought to involve matchmaker AAR, comes as the network providers clash over 4G, with Three this month announcing it would offer the fast mobile broadband service to all its users.
The company was listed second overall in a recent major test of UK network services involving 840,000 checks by RootMetrics, with EE coming out on top and Vodafone bottom.
Angie Moxham, CEO of 3 Monkeys Communications, said: "We’ve had an amazing seven years with Three and feel we’ve been an integral part of its success story. We respect Three wants to review its incumbent.
"We’re politely declining to repitch and wish Three and its new agency partner all the very best for the next chapter in its PR history."
The mobile operator, which is owned Hong Kong’s Hutchison Whampoa, more than trebled its operating profit to £86m in the first half of 2013.
AAR declined to comment.