There are three grades of sacred fire in Zoroastrianism: Atash Dadgah, Atash Adaran, and Atash Behram. The grades indicate the fire’s ritual status and the complexity of its consecration.
Atash Dadgah is the lowest grade and may be consecrated relatively quickly by two priests. It can be tended by a layperson when religious services are not taking place. Atash Adaran, the middle grade, is made by combining fires associated with four traditional occupational groups and requires a longer ceremony involving eight priests.
Atash Behram is the highest grade. Its consecration involves sixteen different kinds of fire, each purified before being joined, and the complete process can take up to a year. A temple housing an Adaran or Behram fire also maintains a Dadgah fire for certain rituals.
The number three refers to grades of temple fire, not three flames in every temple. Zoroastrian tradition also speaks of legendary “Great Fires,” a separate concept that can cause confusion.