Do I get the final cut for free on steam?

My case: I own 1+2 on steam and want to buy part 3 on gog. I do not own part 3 on steam.

No, you'll need to own all three games on the same platform to avail the offer for free Van Helsing Final Cut.

You should write to the developers. I think it's likely that they will give you a Steam key for The Final Cut when you include some proof that you actually own all three games (e.g. screenshots of the receipts).

I rebought 1-2 here just in case :D. I already have 1-2 on steam also, but I do prefer my DRM free games.

First Ask game developers.
May be if youa send them copy of your gog order receipt they agree to send you steam code.
It's highly unlikely though.

Thank you for your answers.

I want to support gog not steam if I have a choice but I don't want to buy games I already own.

I think I try to contact developers and hope the best.

They're definitely not willing to do it the other way 'round.

Mm.. 1+2 from Steam and 3+FC from Gog?
Couldn't that risk some installation mess?
Careful, before ending up with something unplayable..

In what way?

I'm just speculating.
What happens if DRM gets in the way?
Or what if he wants to play FC with Galaxy\Steam and some client-specific pieces are missing?
And about patches and installation checks?

"Yo man, since you own our 3 previous games, here's an extra game for you. Don't mind that the first game is an RTS, the second is a card game, the third is an adventure and the fourth is an RPG."

The final cut will be a separate product from what I recall, so it doesn't matter if you own the previous ones or not. Reason to think that is because people that don't have the previous games can also get it, while those that own some of them get a discount. It shouldn't care about any of the previous games at all.

Ah, I thought that it would have been a kind of upgrade\addon after the 3 games.
Nevermind then.

JMich already gave the answer, but let's hear from the developers themselves:

Q: I don’t own any of these games. Should I buy the individual titles or just wait for the Final Cut? Which one do you recommend?

A: As we have said, it’s better to own the trilogy to get a free copy of Final Cut, but if you choose the buy the Final Cut, you won’t regret that either. It has all the content, it is the definitve version of the adventure and it contains tons of new stuff and features. Also, if you add up the price of the trilogy with all the DLCs, it’s even be a better deal to chose Final Cut. It’s a convenient single installation of the whole trilogy and more. But it’s entirely up to you.

According to the developers, yes you will.

"Anyone who owns the Van Helsing trilogy on GOG or Steam, or GOG and Steam mixed will get Van Helsing: Final Cut for free! No matter what."

Bolded text for emphasis.

Yes!