I purchased GC3 on GOG, and started playing a few weeks ago. I've also read the manual and looked up information online (GC3 wiki and elsewhere) to see what's changed since GC2.
Unfortunately I've found that the game manual is hopelessly outdated and incomplete, to the point it's providing more misinformation than anything useful at all. Since my GOG client is set to auto-update games, I can't say to what degree this desynchronization is due to undocumented changes/updates, or mistakes in the original documentation. I presume it's the former, for the most part, but in the end it all comes down to having bought a game that for practical purposes has no documentation/manual at all!
I'm effectively forced to trial and error to find out what I'm supposed to do, and since many things can only be tested later in the game, this prevents me from planning ahead as I used to in GC2 and other games.
Yes, i can try and find information on the web, but even GC3 is not providing up to date information, and where it does, it's often not clear whether any particular information refers to the base game or requires any or all of the expansions. moreover many concepts are not explained anywhere at all: administrators, legions, special citizens: I've read about them in forum posts, but never found any information on what these are and how they work.
This game is clearly missing an up-to-date source of information for newcomers who haven't read all the patch notes (and postings about additional changes/bugs that aren't even mentioned there).
It's really difficult to appreciate the game when you have next to no information on how it works and you can't trust the information you do find, as most of it is either outdated or requires specific expansions or DLCs to be installed (without mentioning it). The GOG client says I've spent over 100 hours playing by now, but I've spent most of that time hunting for information. Mostly without success. Or trying out stuff only to find out it's not working as described or expected at all.
If not for the many 100s of hours happily playing GC2, I would have dumped this game already. But I know there are some great devs out there who care about the game. So I hope that my call for documentation doesn't go unheard...