Science Fiction: Jack Vance fits neatly into the category of Space Opera, his books are often called "Planetary Romance".
He has written many trilogies and series.
My favourites are :
The Planet Of Adventure series (4 books)
Cadwal Chronicles series (3 books)
The Alastor Cluster series (3 books) (more of a collection; the stories are not linked but share a central common idea)
The Demon Princes series (5 books)
Couple I am not so fond of:
Durdane trilogy
Big Planet series
He has also written many excellent one-off novels, but you insisted that they must be series, so I must discount those. How much I rate Vance is illustrated by the fact that I came across my first Vance novel as a teen and spent years reading almost everything he wrote (around 100 books). Now I am collecting and reading them all again.