The של״ה הקודש notes that the three Parshiot of Matot, Masei, and Devarim, always fall during the three weeks. Any period of mourning is meant to be one of introspection. Each of these Parshiot have its own special message.
All of the commandments are meant to help us perfect our souls, our bodies, and our property. This is what we mean when we say in Shema, בכל לבבך ובכל נפשך ובכל מאודך, that refers to loving Hashem with our souls, bodies, and possessions.
Parshat Matot discusses vows as a means of getting closer to G-d spiritually. Masei discusses murder and cities of refuge that imply caring for the physical body. And Devarim discusses caring for one's property by appointing judges who will enforce the laws regarding property disputes.
All of this is a reminder of the עבודת נפש, the soul work that we are each required to work on. We can improve in spiritual matters, physical matters, and how we deal with our possessions.