The mishap occurred at around 2.30 pm on Doon highway, police said adding the devotees were returning to Biharigarh in Uttar Pradesh's Saharnpur district in the tractor-trolley after paying obeisance at Daat Kali temple here.
The driver of the UP roadways bus tried to overtake the tractor-trolley and hit it from behind on a bridge, causing the vehicle to break the railing and fall into a river.
Three pilgrims died instantly, police said said adding the injured were hospitalised.