The teenagers were walking along Chesterfield Road, opposite the grounds to the Ringwood Hall Hotel, when they were approached by the men at 10pm on Saturday, May 14.
Two boys, aged 15 and 17, handed their phones over.
The three robbers, who were all white, ran off to a black hatchback, which drove off towards Brimington.
Anyone with information should call DC Dave Ingamells at Derbyshire Police on 101.