It's very tough to communicate about User Stories over the internet because there is so much context that can be missed. It's also tough to say whether your splitting is "correct" or incorrect. I think it's also important to point out that User Stories are not a part of Scrum. User Stories are but one technique to represent Product Backlog Items in Scrum, and while it is the most popular method used, it is not the only method. In short, the User Story practice is completely independent of Scrum.
The first thing I would say about your descriptions above is that the sentences you listed are not really user stories. They are "user story descriptions." which means they are 1/3 of a user story. For more on what I mean there, see this article: http://www.scrumcrazy.com/User+Story+Basics
(also see the good user story links at the bottom of the article)
Second, I would ask this question. If you have 3 stories that are essentially the same, is there a material benefit in making them 3 different stories? If so, what is it? (It's hard to tell that without knowing your particular context)
For instance, you could describe your user story as:
Card-Description: "Create Account"
Conversations: <Then have some conversations with the team to help create test confirmations like the below. Hopefully you can have a Scrum team and some users participate in these conversations>
1. Test that an unknown user, a consultant user, and an administrator user can create an account.
2. Test that the user must enter first name, last name, and a email address(with valid email format) in order to create an account.
3. Test that a user is unable to create an account if they don't enter the required information from #2 above.
4. Test that a confirmation email message is sent to the user.
5. Test that the user's new account is not accessible until the user confirms their account by clicking on the link sent in the confirmation message.
> i still have some problem to know when to stop or not with decomposition
This is something that varies with every team, and with how large the stories are in terms of effort. Many teams just starting off with user stories will want to make sure that their stories are no bigger than about 1/2 a sprint. Teams trying to get good at User Stories will often shoot for stories that are a few days of effort, or as few as 2-3 days of effort each.