The period of time we are in now is known as the "Three Weeks." This denotes the twenty-one days between the Fasts of the Seventheenth of Tammuz and the Ninth of Av. (The 17th of Tamuz was the day the walls of Jerusalem were breached, and the 9th of Av was when the Temple was finally destroyed, both the first Temple (c 425 BCE) and the second Temple (69 CE).
Besides for just being a shorter way of saying the amount of time, i.e. 21 days, there is deeper significance to why it is called the "Three" Weeks.
Three is a powerful number in Jewish Law. It reflects permanence and enduring strength. Three times constitutes a "Chazaka" in Halacha. For example, if an animal exhibits certain behavior three times consequetively, it is presumed that this behavior will be repeated in the future. Ownership of a contested property is granted to the occupant who can prove uncontested occupancy for three years. King Solomon the Wise once said: "A three ply cord cannot be easily rent." Three also implies sturdiness. A table of three legs can stand independently, while one of two legs cannot.
Question is, why is this mournful period of time traditionally known as the "Three Weeks?" Certainly this period is a temporary one. The destruction of the Temple began the period of Exile we still experience today, but will soon be terminated by the Redemption, i.e. Moshiach's arrival and the building of the Third Holy Temple.
So why should this time be called by such an imposing number?
The Lubavitcher Rebbe explained that this is no coincidence.
The reason it is known as the Three Weeks is to underscore the profundity of the sadness and the magnitude of the destruction. The fact that it is called three demonstrates the seemingly endless nature of this present Exile, one that has lasted for over 1930 years! The darkness of the present era is unprecedented in Jewish History.
On the other hand, the fact that this Exile has been so long and inexorable also reflects a message of intense hope and yearning.
You see, in the Chassidic perspective, the whole purpose of descent if the resultant ascent -- "Yerida l'tzorech Aliya." Consequently, the unfathomably apparent permanence ("Three Weeks") that characterizes this current exile indicates that the resultant Geulah -- redemption -- from this exile, will be a truly permanent one... an era of eternal and lasting good for all mankind.
Interestingly, the middle Shabbos of the Three Weeks is always the day we read Parshat (Matos) Massei and complete the Book of Numbers (Chumash Bamidbar) in synagogue.
Upon concluding the final words of the Parsha and Chumash, we sing in unison the words "Chazak, Chazak V'nischazeik" ("Be strong, be strong and let us strenthen ourselves!") along with the Torah reader.
Three times Chazak -- three times strength.
Indeed, the Torah gives us the strength to see how Exile is merely a stepping stone to the Redemption, how sadness is merely a catalyst to joy, and the destruction a prelude to the wondrous rebuilding and rebirth that follows!
And this is why the Holy Temple we look forward to is the third one. For it is a truly lasting one.
The Rebbe announced on countless occasions that we are the last generation of the exile and the first of the Redemption. As such, we will certainly witness the rebuilding the Third Temple in our times. May it be in the most immediate future, Amen!