[elmokki]

Dominions 3 is published by Shrapnel. Conquest of Elysium 3 is published by Illwinter (via Desura and whatnot and now Steam). I doubt Dominions 3 will see Steam. Sure, though, I doubted CoE3 getting to Steam too, but reasons were different.

Anyway, at least on Windows installations you can try just clicking the .exe file instead of using Steam to launch the game. That generally works with games that don't use the Steam DRM to it's full extent.