Anyone perusing the SFF section(s) of their local bookshop or library will not be surprised to hear that there are a lot of trilogies and longer series in SFF - if you're new to the genre, or returning after some time away (as I was, a few years back), this can be incredibly intimidating. For a while, I didn't read anything that I knew was in a trilogy or longer series until I knew all of the books had been published - I'd been burned too often by books that took forever to come out, whose publisher had dropped them partway through or where it felt like the author just lost interest towards the end.
So, if you're anxious on the subject, here's 6 suggestions for you to try - these are the first books in either trilogies which are completed (or very soon will be) or longer series with at least 4 books published:
Ancillary Justice by Ann Leckie (trilogy will be complete next month) - once Breq was the mind behind an entire spaceship, now she is trapped inside a single human body and looking for revenge. If you enjoy this book, you might also want to check out the Vorkosigan books by Lois McMaster Bujold.
Rivers of London by Ben Aaronovitch (book 6 due out later this year), published in the US as Midnight Riot - our protagonist, Peter Grant, is a probationary police constable just starting his career in London when he discovers that magic is real and is recruited to the division which deals with it. If you enjoy this series, you may also want to check out the Felix Castor books by Mike Carey, which series starts with The Devil You Know.
The Thief by Megan Whalen Turner (4 books published in this series so far) - Gen is a thief and boasts that he can steal anything, then he's set a task which seems impossible even for him. If you enjoy this series, you might also want to check out Garth Nix's series of books of the Old Kingdom - I'd recommend you start with Sabriel.
Snake Agent by Liz Williams (5 books published in this series so far) - set in Singapore Three, it's essentially an urban fantasy buddy cop series, with Inspector Chen being helped by his demonic counterpart, Zhu Irzh. If you enjoy this series, you might also like T.A. Pratt's series of books featuring Marla Mason, which start with Blood Engines.
The Blade Itself by Joe Abercrombie (complete trilogy, other books being published in the same universe) - this was one of the series that got me back into reading fantasy again after becoming thoroughly sick of the farmboy-prince trope that was everywhere. If you enjoy this series, the other one that similarly got me hooked again was the Gentlemen Bastards series by Scott Lynch, which starts with The Lies of Locke Lamora.
The Hundred Thousand Kingdoms by NK Jemisin (complete trilogy) - an absolutely stunning series with gods and demons and all sorts of thing going on, I would highly recommend it to anyone who enjoys fantasy books. If you enjoy this series, you might also like Elizabeth Bear's trilogy set in a world like Central Asia, which starts with Range of Ghosts.
Happy to receive comments, tell me what you think I've missed and why!