The spokesman said detailed inquiry was in progress and intial findings indicated that the three persons were employed by a contractor firm.
The authorities said that contrary to earlier reports there was no gas emission in the plant.
The plant had been shut down since May eight for maintenance.
District Magistrate S D Chhoten Lama said the bodies were later retrieved from the chamber and sent for post-mortem.
She said initially it appeared that the three had died of suffocation. However, the HPL authorities were inquiring into the exact circumstances of the deaths, she added.
Former CPI(M) MP Lakshman Seth has demanded Rs 5 lakh as compensation each for the bereaved family.
Condoling the deaths HPL authorities said all support would be extended to the bereaved families.
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