We have been waiting for longer than expected, but GSC Game World will be delivering Cossacks 3 [Steam, GOG] for Linux on March 15th.
The developer emailed me to confirm this:
I apologize for not responding earlier. The release date of Linux and Mac versions will be announced today.
If nothing changes, it will be 15th of March. It's a bit far away, but at least I managed to get a release day.
Once again, I'm sorry for the inconvenience.
They have since said the same on Steam, so expect an announcement later.
Update - They have now sent out an email about the update, which additionally confirms this:
QuoteAs a bonus, every Mac & Linux user that purchases and logins into the game during the first two week after the release will receive an Early Bonus package, identical to the one given out during the Windows release.
I'm excited, as we sorely need more traditional RTS games like Cossacks 3