The victim was on a train from Elmers End to Eden Park on November 21, at 7.45pm, when the men sat behind him.
One man next to him demanded his phone and for the victim to sign out of it.
The phone was locked due to multiple incorrect password entries.
All three men made the victim get off the train at the next stop and waited with him until he could try again.
Once outside Eden Park, the victim ran into a local restaurant to seek help and the three men left the scene.
Nearly a month later, the British Transport Police have issued an appeal with CCTV images of three hooded men who may have information that could help with their investigation.
Anyone who recognises them is asked to contact BTP by texting 61016, or by calling 0800 40 50 40, quoting reference 673 of 21 November.
Information can also be given anonymously to Crimestoppers on 0800 555 111.
