רמב"ן — וכפר עליו / וכפר על המטהר מטומאתו / וכפר עליו הכהן וטהר

Q — The metzora brings three korbanot — asham, chatat, olah — and the Torah uses the language of 'kapara' for each separately. Three atonements for one process — why?

A — Ramban: each addresses a different temporal axis. (1) Asham atones for the original 'מעל' that produced the negaim before they appeared (since 'אין נגעים אלא משום חטא,' as Ramban develops at 13:47). (2) Chatat atones for the sin committed during the days of nega — the bitter complaints against Hashem ('נתן תפלה לאלהים') in his isolation; this is why pasuk יט specifically reads 'וכפר על המטהר מטומאתו' — atonement for what was committed during tum'ah. (3) Olah and mincha are 'כופר נפש שיזכה ליטהר ולשוב אל אהלו' — the prayer for return. The summary phrase 'וכפר עליו הכהן וטהר' (pasuk כ) covers the whole eight-day process including the tziporim — which themselves were a כפרה parallel to the שעיר המשתלח on Yom Kippur. (Ramban also notes: house-nega skips asham/chatat because the sin behind it is lighter — body-nega means a graver sin and needs more.)