As a matter of fact the second season of the anime Rosario + Vampire, which was originally published back in 2008, foreshadowed a third season. However, there was never a single sign of any third season, even after eight, long years.
I don't understand why the creators would, as soon as they foreshadowed the upcoming of season three, just suddenly disappear for eight years without notice.
Some fans currently speculate that there will not a third season, as they think that it is over. However, the manga is constantly being updated.
That could only mean that there will be a third season.
However some other sources suggest that there is going to be one soon, either in late 2016 or early 2017. However I'm not sure that their guesses and theories may exactly come true if an active reminder isn't put up for the creators to continue this anime.
