To create a folder within your sub-folder, you will want to open your sub-folder and click the + New button in the top right corner. Choose Folder. It will prompt you to name your folder then click Add folder. You can then add content and files to this third level folder.

See screenshots below

Open a sub-folder and click + New in the top right

Choose Folder

Name your folder and click Add folder

You can then add files and content to this third level folder.

Let us know if you have any questions when building out your data room.