What is the exact definition of VC dimension?
The definition of VC dimension is: if there exists a set of n points that can be shattered by the classifier and there is no set of n+1 points that can be shattered by the classifier, then the VC dimension of the classifier is n. 

The definition does not say: if any set of n points can be shattered by the classifier...

If a classifier's VC dimension is 3, it does not have to shatter all possible arrangements of 3 points. 

If of all arrangements of 3 points you can find at least one such arrangement that can be shattered by the classifier, and cannot find 4 points that can be shattered, then VC dimension is 3.