Sure, let's analyze each set to find the odd one out:

1. A. Once.         B. Three.        C. Two.        D. Four
- **Odd one out: A. Once**
- Reason: "Once" is an adverb indicating frequency, while "Three," "Two," and "Four" are numbers.

2. A. Music.        B. Science.      C. Monday.     D. Maths
- **Odd one out: C. Monday**
- Reason: "Monday" is a day of the week, while "Music," "Science," and "Maths" are subjects.

3. A. Am.            B. Do.              C. Is.                D. Are
- **Odd one out: B. Do**
- Reason: "Am," "Is," and "Are" are forms of the verb "to be," while "Do" is a different verb.