Tzaraas can manifest itself either on the body, on the clothing, or on the walls of the home. The Medrash, cited by the Daas Zekeinem M'Baalei Tosfos as well as the Rambam (Hilchos Tumaas Tzaraas), explains that these three possible kinds of tzaraas are interrelated. They represent a progression of severity that correspond to three different points on the timeline of spiritual decay. At first, Hashem inflicts our homes with tzaraas, in order to encourage us to improve. However, that is just a cautionary shot across the bow. If we heed Hashem's message and see the error of our ways, the tzaraas on the walls of our home will dissipate and not spread any further. However, if we continue down the same sinful trajectory then Hashem will send tzaraas again, this time not to our home but to our clothing, which is more direct and difficult to ignore. If we somehow manage to overlook this warning as well, then the tzaraas will appear on our skin itself, presenting an unmistakable and immediate calamity that demands our undivided attention.
The Kli Yakar observes that the three areas wherein tzaraas can materialize are all coverings. Our skin covers our bodies which is in turn covered by our clothing all of which are housed inside of our home. The Megaleh Amukos claims that the tzaraas found on these three coverings corresponds to the corruption of the three mitzvos which we use to cover ourselves on a daily basis. The Gemara (Menachos 43b) states that Hashem surrounded us with mitzvos in order to deter us from sinning. The tefillin which covers our heads and arms, the tzitzis which adorn our garments, and the mezuzah which is placed on the outside of our home, are all meant to serve as a protective covering and barrier to sin. The tefilin protect our bodies, the tzitzis cover our clothing, and the mezuzah secures our home. Therefore, when one develops tzaraas as a result of habitual sinning, it is appropriate and instructive that the tzaraas manifest itself specifically on one of these three coverings, each of which is safeguarded by a different mitzvah, which was willfully disregarded somewhere along the way.
In addition, the Gemara (Arachin 15b) states that one of the specific sins that most often precipitates an episode of tzaraas is lashon hara, evil and harmful speech. The Gemara continues and explains that in order to protect us from speaking lashon hara, Hashem surrounded our tongue with three covering or obstacles to sin. The tongue is positioned horizontally in a resting position, as opposed to the other limbs of the body which are vertical and poised to act. In addition, the tongue is further shielded by a wall of teeth, which is then enclosed within our lips that can be completely sealed. Therefore, it should not be surprising that there are also three possible forms of tzaraas that can contaminate the individual who presumably spoke lashon hara by breaching the three defensive measures protecting the tongue, and that part of his rehabilitation should include being exiled from all of the three encampments and settlements which encircle and protect the Mishkan (see Vayikra 13, 46).
The fact that our tongue and mouth are so heavily fortified emphasizes the importance of our speech and indicates the potential our mouths have to be a vehicle for kedushah and holiness. In a certain sense, our tongue, which is thrice secured, is comparable only to the holy Ark which was also blanketed beneath three protective layers while traveling, consisting of the paroches, the skin of a tachash, and a cloth dyed with techeiles (see Bamidbar 4, 5-6). Rav Yisrael Salanter explains that Parshas Shemini, which speaks about forbidden foods, is followed by Parshas Tazria and Metzorah which deals with the consequences of speaking lashon hara, in order to teach us that we must be just as careful about what goes in our mouth as what comes out of our mouth. Unfortunately, often, even those people who would never dream of eating something which is prohibited have no qualms about speaking lashon hara. Perhaps for this reason, while often a double seal is sufficient to ensure that food is kosher, our tongue and our speech require a triple covering.