The mishap occurred this afternoon on National Highway 8 when the victims were returning to Surat from Mumbai, Navsari Rural Police Sub-Inspector D C Sakaria said.
"The driver lost control of the vehicle after which it rammed into a stationary truck, killing three on the spot and injuring two children," the officer said.
The deceased have been identified as Girish Mistry (49), his wife Sonal and his mother Hirabai (80), all residents of Surat, he said.
A case has been registered in this connection and investigations are underway, Sakaria said.