State media reports that 40 others were injured when the shaking hit at 11:53 p.m. local time on August 27.
“Two children and an elderly woman were the three [victims] who lost their lives," Iranian Interior Minister Mostafa Mohammad-Najjar told IRIB.
The children were killed at the time of the quake. The woman died from her wounds later at a hospital.
The epicenter was near the city of Damghan, in the Kavir plains, about 175 miles east of Tehran.
The initial quake was followed by about 15 aftershocks.
Residents as far away as Tehran felt the quake, which rattled lamps and ceiling lights in the Iranian capital.
Iran is one of the most seismically active countries in the world. It is crossed by several major fault lines, which have produced many deadly temblors in the past.
Since 1900, at least 126,000 fatalities have resulted from earthquakes across the country.