The boy was found dead in a wooded lot on July 26.
He had been reported missing the day before.
On Thursday, two 25-year-old men were arrested on suspicion of aiding and abetting murder and aiding and abetting kidnapping, writes Aftonbladet.
Since earlier, another man is in custody on suspicion of the same crime.
In addition, two men are arrested on suspicion of murder and kidnapping.