A, some, any

Some
- Used in positive sentences.
- Used in offers.
Example:
1. There are some children at the door.
2. Would you like some water?

Any
- Used like some, but in a negative sentence and question.
Example:
1. Did you meet any friends in town?

No
- Used to mean not any, but with a positive verb form.
Example:
1. I had no food.

Reference: Grammar Practice for Pre-Intermediate Students.