From the Rebbe’s explanations:
– This Shabbos is unique as reflected by the fact that three scrolls are taken out for the Torah reading; we read the weekly portion from one scroll, the Rosh Chodesh reading from another scroll, and the special HaChodesh reading from a third scroll.
This is a very rare phenomenon. There are many occasions when two Torah scrolls are taken out, but taking out three scrolls is extremely uncommon…
The lesson to be derived from taking out a Torah scroll is reflected in the prayers recited at that time which begin:
So it was, whenever the ark set out, Moses would say, Arise, O Lord, may Your enemies be scattered and may those who hate You flee from You. (Beha’alotha, 10:35)
This verse is relevant to every Jew, even in the present era when the ark is hidden in the ground. Every Jew possesses a spark of Moshe within his soul.
This spark brings about “Arise O L‑rd,” an increase in the service of holiness and “Your enemies will be dispersed,…” the nullification of undesirable influences.
Thus, taking out the Torah scrolls reflects both the services of “turn away from evil” and “do good,” the two prongs of our service of G‑d and endows that service with new strength and vigor.
[The “setting forth of the ark” also endows our material concerns with blessing so that we will be able to carry out our service of G‑d without worry or difficulty. This is alluded in the fact that a portion of manna was placed in the ark as “a keepsake for your generations,” teaching the Jewish people that at all times, their material fortunes are dependent on G‑d as they were during the journey through the desert.]
Thus, taking out three Torah scrolls represents a chazakah, a strengthening and reinforcement of the above concepts…
Based on the Rebbe’s talk on Shabbat Parshat Vayikra, Rosh Chodesh Nissan, 5751 (1991). Free translation.