The victim was standing at a pedestrian crossing on Cottingham Road holding his phone, when the suspects approached on what's thought to be a black Yamaha scooter.
They allegedly threatened the man then stole the phone.
The three rode off but the rider lost control and hit the nearby Trinity Methodist church wall.
Two of them ran away while the third rode off on the moped.
The male victim was unhurt.
The incident happened on Tuesday around 7.45pm.
Police are asking anyone with information to call on 101.