This is a bit like asking how long a piece of string should be.
A fantasy work should be as long as it needs to be, no more, no less. One book, three books, ten books, as long as that's how it should be, then great.
Trilogies are a classic in literature, going back to Greek plays. The reason they're so classic is because, at their simplest, they consist of "beginning", "middle", "end", which suits story telling.
Unfortunately, Fantasy is awash with trilogies, so in a sense they've become almost clichéd. This may be because authors think it shows their success in developing a fantasy trilogy like Tolkien and others who've gone before i.e. it makes them "established" in the genre. No doubt there's also an appeal from a marketing perspective, as well as appeal from the author's perspective (having payment/contract for three books). Certainly, from a publisher's point of view, they would be far happier to see three books being written and selling than just one. Readers may like it because, hey, who doesn't want to read more about great characters?
I do think things are changing though. I rarely invest time in trilogies and/or series these days unless reviews are exceptional (for example, The Malazan Book of the Fallen series). For the most part, what I expect are books well-written, and focused. My time is so limited that I simply can't invest the effort into every fantasy series that comes along.
If you feel your story warrants a trilogy, great, but always remember that it's easier to pad and bulk up a story than it is to wield a discerning scalpel and cut out the unnecessary. Ask yourself what is necessary to tell your story.