今日 — Today

今日きょう — the usual way to say “today.”
今日こんにち — meaning “nowadays” or “these days.”

明日 — Tomorrow

明日あした — the usual, casual way: “tomorrow.”
明日あす — often in writing or speeches.
明日みょうにち — formal, used in schedules/notices.

昨日 — Yesterday

昨日きのう — the usual way to say “yesterday.”
昨日さくじつ — a formal way, often in writing.

Tip: These are examples of “irregular” words. They don’t follow the on/kun rule, but they are very common in daily Japanese. Other examples include: 大人おとな — adult, 塩梅あんばい — balance / condition, 海老えび — shrimp. You’ll master these through regular exposure, just like きょう, あした, and きのう.