Eighteen others were also injured in the accident when a lorry hit their vehicle, a police official said.
A mini-lorry carrying the members of the marriage party was parked near the roadside when the other lorry hit it from behind at around 3.45 AM on the outskirts of Mothey mandal in Suryapet district, he said.
One of the victims, identified as Damodar, died on the spot. The groom, Shesha Sainath, and his relatives Satyanarayana (60) and three-year-old Akhil died on the way to a hospital, Sub-Inspector Shrikant said.
The marriage party, comprising 26 people from Kukatpally area of Hyderabad, was on its way to Cherla town of Kothagudem district for Sainath's wedding ceremony, which was scheduled at around 11 AM today, the sub-inspector told PTI.
The driver of the lorry, which hit the stationary vehicle, has been taken into custody, he said.
A case has been registered and further probe is underway into the accident, police said.