The victims, hailing from Ghatampur, were going to their home on the motorbike after celebrating Rakshabandhan last evening when the accident occurred near Ildharpur village, police said.
The truck hit the motorcycle following which the victims fell from it and were crushed under the heavy vehicle, they said, adding they died on the spot.
The deceased have been identified as Chandrashekhar, his wife Rekha and their one-year-old son, they said.
The truck driver is absconding, they said, adding further investigation is on.