I can only get the new races to show up in GalCiv3 if I use the Steam Workshop button in the race selection screen to open the Steam overlay. I then find the race I'm missing, unsubscribe from it, subscribe to it, then click on Click here to return to game at the top of the screen. If that is what you are doing and it doesn't work, here is another possible solution.
When I subscribe in Steam, all the data get downloaded to my computer, but it doesn't get transferred to the GC3Crusade directory in Document\My Games. This is a bug introduced recently, maybe in 3.0 or the last one before 3.0
It is downloaded into C:\Program Files (x86)\Steam\steamapps\workshop\content\226860. If you go there and sort by date with the new ones listed first, you should find the missing race listed first. It will be in a folder with a series of numbers as the name. Open the folder, then open the Factions folder to see the name of the race. If it is the correct one, close that folder. Open the folder Document\My Games\GC3Crusade. Go back to the folder that has a series of numbers as the name. You should see only folders with names like Designs, Factions and ShipStyleSet. You may not get all of these, and you may get more. It depends on what is included with the race. Select all of these folders, right-click in the selected area and choose copy. Select the GalCiv3 folder in Document\My Games, right-click in it and choose paste. If you prefer another way to do the copying, that will work. You will be asked if you want to merge folders, so choose yes.
The race should now show up in the race selection screen.
I have not had to use this method, but I believe it will work. It's just copying data from one location to another that the game uses.