One of the vulnerable teenagers had alerted the police to Tunafors in Eskilstuna shortly after eleven o'clock on Sunday evening.
The teenager and two of his friends had been robbed by three people a few minutes earlier.
During the robbery, one of the teenage boys is said to have been hit in the face.
- I do not know what they were robbed of but there was a robbery and also an attempted robbery.
The suspects must also have threatened their friends not to turn to the police, says Dennis Johansson Strömberg, the police's spokesperson.
One arrested - the police are looking for more
During the robbery, one of the teenage boys is said to have received a slap in the face.
However, the person should not have had to seek medical care.
The police searched the area where the incident is said to have occurred and were able to arrest a suspected perpetrator.
He has now been arrested on suspicion of robbery, attempted robbery, harassment and assault in court.
The man has already been convicted.
The police are now looking for the other two perpetrators and are asking witnesses who saw something suspicious in Tunafors on Sunday evening to get in touch.