The Lubavitcher Rebbe explained:[1]
The time of the breaching the walls of Jerusalem on the 17th of Tammuz until the actual destruction of the Temple on the 9th of Av encompasses three weeks.
The number three is a number that is expressive of permanence and solidity.
This hints that the inner intent of the destruction of the Temple was for the greater edifice that will be in the third Temple—this is congruous to the rule that all descents are for the purpose of ascension.
[1] Sefer HaSichos 5751 Vol. 2 Pg. 584
