Two labourers - Vijayendra S. Bagdi, 36, and Jagdish Parmar, 54 - fainted and collapsed in Goregaon on Wednesday evening. They were rushed to nearby hospitals but were later declared dead.
A 24-year old youth, Sharukh S. Shaikh, drowned in the Mahim Creek near Kurla and his body was fished out by the Mumbai Fire Brigade later in the evening.
One boy, who had jumped into the swollen Mithi River for a swim along with three friends, is still missing, while the Indian Navy has launched a search for him since Thursday morning.
The authorities recovered the floating body of a 60-year old man, Ashok D. Mayekar, near the heavily flooded Hindmata area in south-central Mumbai.
Releasing the rainfall data of September 4, the BrihanMumbai Municipal Corporation (BMC) Disaster Control said the city recorded 127.9 mm (season's total - 2,263.2 mm), the eastern suburbs got 173.4 mm (3,006.3 mm) and the western suburbs notched 186.0 mm (2,765.1 mm).
The average for the city and suburbs stood at 102.73 mm and 114.79 mm, respectively, with the season's total averaging to 2,358 mm, or nearly 114 per cent of the annual rainfall.
At least 15 locations in Mumbai received in excess of 20 cm and 30 cm of rainfall within 24 hours on September 4, as recorded by the BMC's rain gauges in different areas.
They include: Vikhroli East (35.4 cm), Borivali (33.0 cm), Marol (31.4 cm), Wadala (31.0 cm), Kandivali (30.3 cm), Dharavi (30.0 cm), Dadar (29.4 cm), Andheri East (28.5 cm), Vile Parle (28.0 cm).
In view of the heavy rains which flooded the Mithi River, rescue agencies shifted around 1,400 people to safety in Kurla's Krantinagar and Bail Bazar areas where water seeped into their homes.
Another 2,300 people stranded at various locations in the city, including railway stations, were put in temporary shelters in different parts of the city.
The IMD has forecast heavy rains for Mumbai, Thane, Palghar and Raigad districts on Thursday, and people have been advised to keep away from the stormy Arabian Sea, beaches and creeks.
After Wednesday's holiday, Education Minister Ashish Shelar declared a second day of precautionary holiday for all schools and junior colleges in these districts.