The three locations where the priests stood watch were the Chamber of Avtinas, the Chamber of the Spark, and the Place of the Fire.
The first two locations were structures built into the side of the court like upper lofts:
In The Chamber of Avtinas the incense which was offered up on the golden incense altar in the sanctuary was prepared, and it was called after the priestly Avtinas family, who were entrusted with the sacred task of compounding the ingredients and creating the incense for the service.
According to descriptions in the Jerusalem Talmud, this chamber was located on the south side of the court, directly over the "Water Gate" (called so because it was through this gate that the golden flask of water was brought up to the Temple from the Shiloach on Sukkot, during the Festival of the Water Libation).
The Chamber of the Spark was located in the north of the court. Here, a small fire was kept burning to provide the fire which burned perpetually atop the altar.
The watches in both the Chamber of Avtinas and the Chamber of the Spark were manned by young priests who had not yet reached the age of their official Temple duties.
The Place of the Fire was likewise situated on the northern side, east of the Chamber of the Spark. This was not an upper loft, but a very large room covered with a domed roof. A great fire was kept burning here all the time, and some say that the officiating shifts of priests would warm themselves around it. As they performed their tasks barefoot on the bare marble floors of the Temple, they were subject to cold. But most opinions hold that the priests who returned from immersiing themselves for purification would afterwards warm themselves in this chamber.