The gemarah Yerushalmi in Meggilah says that just like the Torah was given to Klal Yisrael from Hashem through a middleman (Moshe Rabbeinu), so too whenever we read from the Torah it should be with a middleman. The person reading the Torah is the middleman and the person called for the aliyah is in place of Klal Yisrael receiving the Torah. This halacha is brought by the Shulchan Aruch 141:4. The Levush adds that a third person should stand by the bimah. Just like Hashem gave the Torah to Klal Yisrael, this third person should tell the ba’al koreh when to start, as if giving over the Torah.
The Mishna Berurah quotes this as a reason to have three people at the bimah, but does not specify who should stand in which place. The Aruch Hashulchan says that the person getting the aliya should stand in the middle, the ba’al koreh on the left, and the gabbi his right.The Mishneh Berurah also gives a second reason, that the three people are symbolic to the three Avos. He adds though that the ba’al koreh should be in the middle and the gabbi to his left.
Different congregations have its own custom regarding which side of the bimah the gabbi should stand. Some try to satisfy both opinions and have one gabbi on the left and one on the right of the bimah. If however the gabbi or the ba’al koreh has the first aliyah, leaving only two people standing at the bimah, a third person should come and stand for the duration of that aliyah.

Rabbi Tzvi Aryeh Hyman, Mir Yeshiva
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