noun
morning, noon, and night; all day long
Refers to the three main parts of the day as a set, often implying 'throughout the day' or 'from morning till night'.
朝昼晩と働きづめだ。
I work nonstop morning, noon, and night.
朝昼晩、この薬を飲んでください。
Please take this medicine morning, noon, and night.
noun
breakfast, lunch, and dinner; three meals a day
Abbreviation for 朝ご飯・昼ご飯・晩ご飯. Used informally to refer to the three daily meals as a set.
朝昼晩、ちゃんと食べてる？
Are you eating breakfast, lunch, and dinner properly?
朝昼晩、外食だとお金がかかる。
Eating out for all three meals gets expensive.
三食 is a more formal term for 'three meals', whereas 朝昼晩 is a casual, colloquial abbreviation.
Compound of 朝 (あさ, morning), 昼 (ひる, noon), and 晩 (ばん, evening). The reading ひる becomes voiced to びる due to rendaku. The second sense is an abbreviation of 朝ご飯・昼ご飯・晩ご飯.