Hostel 1 and 2 benefited from being set in Eastern Europe, grey concrete and sinister people somehow setting the films apart from the very beginning.
Hostel 3, though, is much like many other horror films: a group of young guys on a bachelor party trip to Las Vegas meet a couple of girls. They think they're going off to a club, but end up in cells, prisoners of this "hunting club", a group of rich Americans who like to watch people die and place bets on exactly how and when it'll happen.
And so a few people are killed in various ways, only one of which is fairly gory (the others are either relatively ordinary, or in one case quite amazingly stupid). Tension ought to be building here, but for most of the film it really isn't: in part this is because most of the characters are unlikeable and you won't care what happens to them, but also in quite a few cases it's fairly obvious that the person you're watching is going to die, the only question is whether it's now or in 30 seconds time.
And the "twist" ending doesn't help, either, or at least it didn't for me - I found it totally unbelievable.
So should you ignore Hostel III completely? Well...
The first 5 minutes of the film were good. Not great, but good - I can't tell you why, but if you're a fan of the series you'll appreciate it.
One of the kills is quite gory. There are also one or two interesting twists in the second half of the movie. And as we get to the end there are moments where things do get quite gripping.
For most people, then, I'd say leave Hostel III alone. There are too many stupid moments, and those are the ones that will stick in your mind. But if you're a big fan of the series, and want to see what happens next, and you set your expectations just about as low as they can be, then there are also one or two moments you'll enjoy here. Not many, and not enough, but also it's not the total disaster that some others say.