Police said the accident happened at 7am at an intersection in Moo 3 village in Tambon Nong Ong in Uthong district.
The mother, La-ongsiri La-ongnual, 41, and her daughter, Arinlada La-ongnual, 15, died on their way to a hospital. Siwakorn Chaokongjak, 14, was severely injured.
Police arrested the truck driver, Chaiwat Permpool, 45, on a charge of reckless driving causing deaths and injuries.
Police alleged that Chaiwat made a right turn at the intersection without first checking that the way was clear. He collided with the motorcycle driven by La-ongsiri.