The incident occurred in Khaista Baba Kotkay area of Buner district bordering Swat in northwest Pakistan's Khyber Pakhtunkhwa.
The shepherd brothers aged 17 and 14 years and their 10-year-old sister found the bomb while grazing their animals in the mountainous area and started playing with it before it exploded, an official said.
The boys died on the spot while their sister was immediately shifted to a nearby hospital, the official added.