For digital game, don't buy directly on Steam it's the most expensive option. I went with GamersGate with a code as it was 1/5 lesser price. Deep Silver suggests two options (shop menu at risen3.deepsilver.com), one is HumbleStore which is also full price, but another is 10% less price so that's still not a bad option.
Key resellers have prices less than these, for example go2arena offers preorder for 26€ but I don't buy preorders on such sites. Only old games.
Why?
Let's try to think how these sites operate. Older games were on sale somewhere we didn't see or cared in the past. They bought a bunch of copies, added 10%-20% on the sum they've paid and now they resell those keys when those games are not on sale any more. And we get them for cheaper price, everyone's happy. Perfectly logical, right?
But how can they resell preorders then? It's shady and not logical thus I avoid that.
I did buy keys on mentioned go2arena site, every single key worked and for that part that site is safe. But I can vouch only for older games, I can't say for sure their "preorder" key will work! Why I say "preorder"? Because it doesn't include real preorder day1 DLC.
For CE versions, languages and mess up please be a bit more patient. A moderator on DS forum also got confused after I gave some data on it but promised to look at it some more today. If he can't clear it up, well, there is always Deep Silver support.
@JDR
No key yet, they have to receive those from Steam first. Expect it a few days prior to release. I seriously dunno how the code didn't work on you. Maybe you used some that expired?