Cow was taken on in mid-2014. The brief had been handled by 3 Monkeys prior to that.
A Three spokeswoman said: "As part of a wider company review and revamped marketing strategy, we have decided to put the consumer PR agency support out to tender."
Three welcomed new group marketing and creative director Shadi Halliwell, previously of Harvey Nicholls, in September. She had spent more than two decades in the mobile industry prior to Harvey Nicholls.
The spokeswoman continued: "We have already shortlisted and invited a select few agencies who we’d like to consider. We want to thank Cow PR for all their work and support during the past four years."
Three uses Headland for corporate and financial PR - the agency was hired in May last year to an expanded brief, with incumbent MWW losing out.
Cow founder and MD Sian Morgan said: "We've had a blast, done some absolutely cracking work and increased Three's coverage levels by over 100 per cent. We’ve very proud of what we achieved for the brand but now it’s time to hand over the baton."
Elsewhere, Cow has been taken on by BFree Foods, a food brand containing no gluten, wheat, dairy, egg, nuts or soy, on an initial six-month project to raise awareness of the benefits of such food across a broad consumer audience. BFree joins a health and wellness division that also includes gym chain David Lloyd and another recent win, online nutrition retailer Healthspan.