Police officers were called at around 4.10pm on Sunday, September 9, following reports of the incident in Peplins Way, Brookmans Park.
A 16-year-old boy was allegedly approached by a group of people who asked to use his moped, before one of them revealed he had a knife, a police spokeswoman said.
After unsuccessful attempts to take the moped, the group then left. Officers then searched the area, helped by the police helicopter and dog unit, before three teenagers were arrested on suspicion of robbery.
A police spokeswoman confirmed they are all in police custody and the victim was not injured.
Anyone with information can contact Detective Sergeant Dean Trollope via the non-emergency number 101, quoting crime reference 41/35269/18.