Magix is a magical planet as well as the name of the major city located on it. Magix is located at the center of the Magic Dimension, making it its capital, as well as the "Crossroads to All Magical Dimensions." Due to the three major schools being located on the planet, Magix acts as a major setting for most of the Winx Club series.
These three schools are the largest and most well-known magical schools of Europe, similarly to how Alfea, Cloud Tower and Red Fountain are the largest and most well-known magical schools of the Magic Dimension.
Durmstrang has a reputation for Dark Magic, similarly to how Cloud Tower's own reputation for dark magic.
Beauxbatons is an all-girls school (In the films, while in the books it's not a all-girls school) much like Alfea and Cloud Tower.
Alfea (above), Red Fountain (middle) and Cloud Tower (below)