For some reason it's not included in the gog galaxy version of popTB,
To get the expansion, you will need to download the game manually through the web client.
You mean thru Galaxy? When I look on the GOG product web site and click More -> GOG download links, I don't see anything more.
Hey ! i have had this problem too ) i found undescovered worlds game in gog galaxy
so steps to get this game :
1)go to gog galaxy
2)find your populous 3 game (click )
3)find (and click ) button MORE which near PLAY buton
4)find backups and additional materials (click)
5)find backup file of populouss 3 and click
after that game would be downloaded to direction what you choice
and that file would have populous3 and addon UW ..you need only to intall it and play
PROFIT ^ ^ ARATANGA lol