Andy Cornelius and Duncan Murray will receive equity at College Hill, becoming partners in March.
Justin Walker rejected College Hill’s overtures, but is believed to be staying within the industry.
The move will see the openly acquisitive College Hill take over some of Gainsborough’s biggest clients, including department store Debenhams, car and bike equipment retailer Halfords and supermarket Somerfield.
Gainsborough received two or three approaches, but opted for College Hill after discussions that began late last year.
Cornelius, Murray and Walker are all former Citigate Dewe Rogerson directors.
Prior to that, Cornelius was a financial journalist for more than 20 years; Murray held senior roles at Taylor Woodrow and Associated British Ports; and Walker was European comms head for Chase Manhattan Bank.