Construction of the St Peter's church near the new bus-stand was on for the last few months and concrete work on its roof was going on last night when the scaffolding gave way leading to the wet roof caving in, police said.
Workers were trapped by the concrete mixture and steel, they said, adding, three of them died on the spot.
The injured were admitted to hospital.
District Collector M Karunakaran visited the injured at the hospital.