Ramachandran was previously one of four creative directors at Freuds, with a focus on digital and innovation.
He starts at Three today, reporting to director of propositions Jasmine Skee, who in turns reports to Three's CMO. His role is focused on creating new products and services to attract customers to the network.
Ramachandran had joined Freuds last year from M&C Saatchi Sport & Entertainment.
Skee previously worked for O2, during which time the mobile firm was a client of M&C Saatchi Sport & Entertainment, Ramachandran's then employer.
Ramachandran has also worked for Splendid and Text100.
He said of the attraction of the new role: "The appeal of Three is that the business is marketing-led, from the CEO down they respect and back their marketing team - that's not true of every business. They're also a business that is prepared to take risks."
He cited a recent Snapchat campaign as an example of this - Three is now allowing all customers to use Snapchat without it eating into their data allowance, and has created a Puggerfly that users can care for in a similar way to a Tamagotchi via the platform.
Freuds CEO Arlo Brady said: "Haran played a significant role in developing freuds digital and creative offer and we wish him all the very best in his new role."