The 3 C's is a saying used to serve as a reminder on what is important when using User Stories.
A simple card to write down the requirements - as opposed to a heavy weight document.
Having a conversation - collaborating to define the requirements and understand the value.
Confirmation - agreeing on the acceptance criteria, so you know when you are done.
There is another saying which sums up the idea behind User Stories nicely. "The card is a place holder for a conversation". This highlights that the conversation is the important part, not the card artifact. The card can be discarded once the feature has been developed and automation tests written using any suitable acceptable criteria.
The format of a User Story will typically follow:
As a (role) - This can be an end user or a business proxy
I want - A description of what need to be done
So that - the definition of the value
An attempt at your scenario
As a vending machine customer
I want my change returned
So that I do not loose my money
The confirmation part could just be bullet points on the back
- Change should be returned if the customer does not put in enough money and then selects a beverage
Or you can use the context specification style, a BDD (Business Driven Development) technique
Given a customer does not put in enough money
When they select a beverage
Then their change should be returned
If you would like to know more I would suggest researching the INVEST principles and reading Mike Cohn's User Stories Applied.