Authorities say they received reports of a structural collapse at the site of the old General Motors plant on Linden Avenue in Linden just after 7 a.m.
The three victims, one of whom suffered a serious head injury, were reportedly struck by steel girders.
All three were taken to University Hospital in Newark.
The site was immediately closed down, and the cause of the collapse is under investigation.
City construction officials and OSHA have been notified to respond to the scene.