The incident took place at 10 AM in the Dombivali MIDC area killing the trio on the spot, they said.
The explosion also triggered a fire which was later put out.
Among the dead were two workers and a car driver identified as Kalias B Rajbhar (55) Vijay R Gaur (40) and Mahilang Yadav (35), Suresh Shinde, the Station In-charge of the MIDC fire brigade at Manpada, Dombivili, told PTI.
While two of the workers died on the spot, the driver was hit by a heavy blown-away boiler part, killing him.
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The cause of explosion is still being probed by the police and firemen, DDC officials said.
The injured have been hospitalised.
The impact of the explosion and fire was such that the glass panes in the near by houses broke.