The incident took place on a road in Moo 3 village, tambon Chalerm just before 4pm.
Security officials called to inspect three bodies identified the two men as Ismael Dawrauh, 35, and Assanai Jaeso, 32. The name of the young girl is not yet known.
The victims were travelling home on a pickup truck after running an errand in the village when at least two gunmen hiding in roadside bushes sprayed bullets at the vehicle from AK-47 assault rifles.
Ismael was hit by multiple shots to the torso and head and died instantly. The assailants then came out onto the road and fired more shots at the pickup. One bullet hit the girl in the chest, killing her, said Pol Col Jiradej Phrasawarng, chief of Rangae police station.
Assanai was injured in the attack but managed to flee into a roadside bush. The gunmen shot him in the back twice before leaving the scene.
Police found 12 spent cartridges from AK-47 rifles at the site, Pol Col Jiradej said.
Investigators blamed separatist militants.