That this interval on the calendar is known as the “Three Weeks” and not the “Twenty-One Days” is not incidental. The number 3 alludes to the inner significance and function of the Three Weeks as a period of preparation for the Third Holy Temple. On a superficial level the Three Weeks are a sad time, a period of mourning for the destruction of the Temple and the beginning of the Jewish people’s current exile. But on a deeper level they contain a hidden good. Why?
Everything that happens in the world is directed by G-d. G-d is the essence of good, and everything He does is good, even if it doesn’t appear that way at first. Having come directly from G-d, there is no other possibility. Accordingly, the Three Weeks, although superficially associated with sadness, contain a positive meaning:
At the exact moment when the Second Holy Temple was destroyed, the Third and eternal Holy Temple was constructed up in heaven! In this light the entire destruction can be seen as nothing but a preparatory stage in the Redemptive process, a necessary step toward the Final Redemption with Moshiach, at which time the concept of exile will no longer exist. At present, the good contained within the Three Weeks remains hidden. But reflecting upon its true, inner meaning hastens the day when its inner goodness will be revealed, when the Temple will be reestablished.
Let us therefore accustom ourselves to seeing the hidden good that exists in all things, thereby meriting the ultimate revelation of inner goodness with the arrival of our Righteous Moshiach. ~ Lubavitcher Rebbe zt”l