Police will allege three teenagers murdered Paul Costa in a Brunswick West reserve in a carjacking gone wrong.

His body, clad in an Ermenegildo Zegna suit, was found by a passer-by in Dunstan Reserve, a popular dog walking spot and sporting ground at Brunswick West, just after 8.20am on July 3.

Homicide squad detectives arrested a 17-year-old boy and an 18-year-old man in Thomastown on Thursday morning.

A 16-year-old was arrested in the NSW city of Newcastle and will be extradited to Victoria on Friday.