Hi, I downloaded three custom structures from this site and placed them in the themed folder of RCT3 platinum. Attempted to build the custom stuctures in game and they aren't showing up in the file. Please help

You need to place them in documents/rct3/structure not style/themed.
If any CSO is used that needs to be downloaded and placed in style/themed

Hope this helps

Sorry, I can't tell what you mean, I'm on mac. Do I need to create a folder in documents labeled rct3? I tried and it didn't work

Okay so this video shows you the place where you actually need to place the structure file you downloaded. The video is actually for retrieving old deleted parks, but in the second to last folder he enters is where the place where the custom structures folder and custom coaster folder is. If this doesn't help please PM.

https://www.youtube.com/watch?v=D5V_uxI0MRM