Phillimore will remain the 'face' of Cornbury as the festival creative director.
He revealed: "I'm very excited to be working with 3a. I’ve found the perfect partner who thinks like us and is independently minded. They’ll bring to the festival all the things we’re missing and they have the integrity to leave us to get on with it, so the unique nature of the festival won’t change."
Created in 2004, the event has seen acts such as Paul Simon, Amy Winehouse and Robert Plant all perform.
3a director Pete Wilson added: "Having created the Route of Kings concert series in Hyde Park, we understand the special place Cornbury occupies in the festival landscape and we look forward to nurturing it for years to come."
For 2010, the festival often referred to as Poshstock, will move to a new location ten miles from its original site in Oxfordshire.
For an exclusive interview with Phillimore make sure you check out the July/August issue of Event magazine. You can subscribe