Site structure - nodes and blocks I'm new to drupal and I try to understand the structure of it.

Let's assume I want to do the following regardless of the existence of a module doing this concrete job – just to understand the structure and best practices.

What I need is a calendar module. This calendar should be displayed on an own page. So I think I must provide a special calendar content type, don't I?
Then I want to display the next 3 events as a list on a front page. Here, I guess, I must provide a special block, which is only visible on the defined front page?

Is this the standard way of doing it?
Your question is hard to answer because there are calendar modules.  How those modules are implemented could vary infinitely. 

Drupal being so flexible you could pose this question to 10 people and get 10 different answers and they would all work!

If I were to have this problem I would look for a calendar module that would expose the data it may contain to Views so I could do anything I wanted with the data.  That would mean that the calendar module would use the Entity framework to save it's data which is the best way of doing things in Drupal.

The display of the calendar more than likely would not be a content type, it could be content you could insert into a content type using tokens or something like that or it may supply you a page and path to the calendar view or multiple paths.  Again I go back to you could pose this question to 10 people and get 10 answers.

This may not answer your question.  But your question is rather hard to answer without specifics in regards to modules.