Building No 14 at Arihant Complex in Narpoli area came crashing down around 1.15 am today, said investigating officer Ramesh Patil of Narpoli police station.
Two dozen people managed to escape, while 15 others were rescued and admitted to various hospitals for treatment, Patil added.
The dead were identified as Virendra Pandey, Sheikh Sadamul Lal Mohammed and Munna Diwan, all working in garment factories in the powerloom town, situated around 35 km north of Mumbai in Thane district.
The cause of the collapse is not yet clear and rescuers are working to save at least eight other people suspected to be trapped under the debris.
The building is said to be around five years old and a portion of it was still under construction, Patil added.
This is the latest in a string of building collapse tragedies in the Mumbai metropolitan region in the past three weeks.
An old four-storey building collapsed at Mahim in central Mumbai June 10, killing 10 and injuring six people.
Ten people, including six children, were killed June 21 when a 35-year-old three-storey building crashed in Mumbra town in Thane district, about 40 km from here.
The next day, June 22, seven people were killed when an abandoned building collapsed following heavy rains in Dahisar suburb of north Mumbai.
Kalyan Shilphata near Mumbra had witnessed the state's worst-ever building crash April 4, which killed 75 people and left 60 others injured.