Hey, I know this is an old topic, but after looking for "sc3kaddonplug-in.exe" on many sites - SimCity "webarchive" did not help - I found the original installation file on a Polish site.
http://chomikuj.pl/michalszasz12/sc3kaddonplug-in,1309083077.exe
It worked fine here, no error messages after setup. I have the original CD version (sc3k and classics), not gog / unlimited. I do not really know what "SC3_Plug-In_1.Exe" is for, but it came with the original reference package that I downloaded here on another topic. I installed all packages anyway.
I figured the problem of the DLL error message was something related to the version of the file, and I noticed that the "sc3kaddonplug-in.exe" of the original site had 650KB, and the file "SC3_Plug-In_1.Exe" has more than 900 ... so since I have the original version of the game, I figured it would be better to try to find the smaller installer. Maybe I am wrong, and there is no relationship between the two, just know that I initially ran the game smoothly when installing a new building just with "sc3kaddonplug-in.exe".
Anyway, I hope it helps other people who want to play the original sc3k again !!!!
update:
1 - I finally found a working mirror for downloads on wayback machine and it hosts all original landmarks for sc3000 and the plugin: http://web.archive.org/web/20010616034855/http://SimCity.ea.com:80/us/simexchange/downloads/sc3k/index.phtml
2 - Now I see the difference between "sc3kaddonplug-in.exe" and "sc3_plugin_1.exe", I didn't pay enough attention.
