The three were rushed to a nearby hospital, where two of them were identified as Pornchanok Polkrajang, 14 and Panida Panmai, 17.
The driver of the van, which was carrying 24 students at the time of the accident, claimed it was caused by poor visibility due to the thick fog.
His name was not known.
According to a weather report from Phitsanulok Meteorological station on Thursday, the town was encountering thick fog tipped to last until tomorrow.
Meanwhile, the Meteorological Department said the temperature in the North was expected to drop by seven to nine degrees Celsius from Monday to Wednesday.
The department said the drop in temperature was due to the influence of a westerly trough from Myanmar, which is now covering the North.
Lowest lowland temperatures in the northern region ranged from 18 to 22 degrees yesterday and from 5 to 10 degrees Celsius on mountaintops.