Anant Ram (40) a resident of Nepal, Dhniyal (36) and Vijay Kumar (27) of Jharkhand died on the spot, whereas the six injured were admitted to regional hospital Hamirpur, police said.
The incident occurred when the work of laying the slab was going on at the GIS Apartment of the Power Grid Corporation at a height of 35 feet, they said.
Soon after the slab was laid, it collapsed killing three on the spot, they said.
The construction work was being carried out by a local contractor of the district.
A case has been registered and the matter is being probed to ascertain the cause behind the incident, ASP K G Kapoor said.
