Let’s see it in more detail.
Number 10 represents a state of perfection (and thus the Era of Redemption is associated with the number ten).
Number 30 (3×10) is a chazakah of that perfect state.
Number 90 (3×30) is a chazakah of that chazakah, and thus the fullest possible expression of the idea of strength and permanence.
Remarkably, there is an intrinsic connection between Moshe and the concept of permanence as reflected in our Sages statements: “Moshe did not die” and “the work of Moshe’s hands is eternal.”
In this context, it should be noted that the 90th Chapter in the Book of Tehillim is a song written by Moshe Rabbeinu, as it says in the opening verse:
“A prayer of Moses, the man of G-d” (Tehillim, Chapter 90).
Our sages interpret this as a prayer in connection with the construction of the Mishkan built in the desert. With this prayer, Moshe permanently established the dwelling of Shechinah (the Divine Presence) among the Jewish people.
For more chassidic insights, please, see the selected excerpts from the Rebbe’s commentary on this chapter.