TT #254 · Parshas Tazria · Aliyah 7 · Passuk 46
English: All the days that the affliction is upon him he shall be impure; he is impure. He shall dwell alone; his dwelling shall be outside the camp.
Torah Temimah cites Sifra (Toras Kohanim) for excluding a metzora from all three camps.
Sifra — Vayikra 13, Midrash Sefer
English: “Outside the camp” means outside all three camps.
Textual derivation
The Sifra defines the metzora’s exclusion as extending beyond all three camps.