Other books that are doing well in this sort of genre, with this type of audience tend to be quite monochrome -- heavy black and white, with a splash of colour to set it off. I'd be happy to go with a sort of black and white, dark and horror-esque sort of cover with a solid but understated blood-red in there to give it a bit of a kick. Usually, the titles of these books are in a thick sans serif font white on a black background just to give them a bit of pop in the ads.
Seeing as this is set in Oslo and Norway, I'd be happy to see some pine or fur trees if you wanted to go eerie, but preferably something more than just a stock photo of a forest with text over it.
There's a pivotal scene where they're travelling a road to Oslo and hit a huge concrete wall that's been erected around the city, so if you want to take cues from that you can. Otherwise, there are biopunk influences, so anything sort of biological could be cool.
Or, in the opening, some bad stuff happens at the protagonists house in a quiet suburb. He comes out in the morning to find peoples doors all smashed in, windows broken, police cars abandoned, that sort of thing. A front door broken on its hinges, some claw marks (understated if possible), or a street like above might be interesting, though it's hard to say how this sort of thing will look or play. Check out the inspiration images of the charts above and note the ones I've mentioned for stylistic cues and then see if anything comes to mind.
The 'monsters' in the books themselves are a hybrid between a wolf and a lizard (bear with me), and while there's a long explanation to why this is in the book, they're not really referred to in all that much detail. The terror is in the unknown I guess. But either way they're big, black, teeth and claws, with snake-like eyes and long tails... I'd be happy to keep them off the cover in all honesty, but if you can subtly work it in and you think it'll make a good addition, then go for it.
At the end of the day, the narrative is three parts from three perspectives, so I think something dark, unnerving, and abstract might be a good way to go so the cover isn't too focused on one person. But then again, a silhouetted figure facing away seems to be a popular trope. But hey, you guys are the experts, so I'll leave it up to you. Keen to see what you come up with!
There'll be three books in the series, so whoever I choose now will need to be able to design the sequels in the same style.
I have a site that I've been putting together for the author (penname), and I'll include some screengrabs from it so you can get a sense of the sort of mood I'm looking to strike. But don't take too heavy cues from it. I'd rather see you guys do what you do best!
Any questions, please get in touch. I can't wait to see what you come up with.