The trilogy is Morland by Peter Schwindt and the three books are named Die Rückkehr der Eskatay, Die Blume des Bösen, and Das Vermächtnis der Magier.
It seems like the books were written in German and never translated.
German Goodreads summary of the first book:
"Verschwinde!", kreischte Phineas und hob abwehrend die Hände. "Ich will dich hier nicht mehr sehen, hörst du? Du bist ein gottverdammter Eskatay!" Er spuckte dieses Wort regelrecht aus. "Ein Eskatay?", fragte Tess hilflos. "Was soll das sein?" "Ein Teufel in Menschengestalt! Das Böse schlechthin." Phineas' Gesicht war zu einer Fratze verzerrt.
Vor vielen Tausend Jahren vernichteten die Menschen Morlands alle magischen Begabten. Sie fühlten sich sicher. Doch nun sind die Eskatay zurück.
Google translated:
"Get out!" Phineas shrieked and raised his hands defensively. "I don't want to see you here anymore, you hear me? You're a goddamned Eskatay!" He practically spat out the word. "An Eskatay?" Tess asked helplessly. "What's that supposed to be?" "A devil in human form! Evil par excellence." Phineas' face was twisted into a grimace.
Many thousands of years ago, the people of Morland destroyed all those gifted with magic. They felt safe. But now the Eskatay are back.
After thinking about it for some time I remembered the title and searched for images on google until I recognized the cover.
