1. It is fifteen past three.

2. It is fifteen after three.
3. It is a quarter after three.
4. It is three quarters after/past two.
(Are they all the same and grammatical?)

5.It is ten before three.
6. It is ten minutes before three.
7. It is ten minutes before three p.m.
8. it is ten minutes before three o'clock.
9. It is ten before three p.m.
(Are they all grammatical? Which ones are not used?)

All are grammatically correct, but not all are commonly used.

These are the uncommon ones:
4, 9

1. It is fifteen past three. (Grammatical)

2. It is fifteen after three. (Grammatical)

3. It is a quarter after three. (Grammatical)

4. It is three quarters after/past two. (Grammatical)

All of these phrases are grammatically correct and convey the same meaning, indicating that the time is 15 minutes past the hour of three.

5. It is ten before three. (Grammatical)

6. It is ten minutes before three. (Grammatical)

7. It is ten minutes before three p.m. (Grammatical)

8. It is ten minutes before three o'clock. (Grammatical)

9. It is ten before three p.m. (Grammatically incorrect)

The first eight phrases are grammatically correct and are used to express that the time is 10 minutes before the hour of three. However, the ninth phrase, "It is ten before three p.m.," is not grammatically correct. When indicating time, it is more common to say "ten minutes before" rather than "ten before" when specifying the number of minutes.