Dyfed-Powys Police said thy are investigating the incident, alleged to have taken place at Back Lane, Newtown, at around 4am on Sunday.
A spokesman for the force said: "The victim was assaulted by three males, one of which is believed to have been in possession of a knife. A 37-year-old man received hospital treatment but has since been discharged.
"Three men have been arrested in connection with this incident and have been released on conditional bail whilst the investigation is ongoing.
"We are not currently looking for anyone else in connection to this incident and are grateful for the continued support of the public in assisting with our enquiries.
"We would still urge anyone with information that could help officers with their investigation to report it to Dyfed-Powys Police, either online at: https://bit.ly/DPPContactOnline, by emailing email@example.com, or by calling 101."
Alternatively people can contact the independent charity Crimestoppers anonymously by calling 0800 555111, or visiting crimestoppers-uk.org.