Using symlinks or junctions on drives or folders, you will still have 3 sub-folders rather than merging all the files into a single folder.
One suggestion - instead of creating a symlink/junction for each of your drives, create the symlink/junction for the actual sub-folders where the files are. This will mean only one sub-folder to navigate to get to your files.
For example, this creates a new folder on your C drive and create 3 junctions for your 3 folders where your files are:
md "C:\TV Series"
mklink /J "C:\TV Series\Q" "Q:\TV Series Complete Q\TV series Folder\files"
mklink /J "C:\TV Series\Y" "Y:\TV Series Complete Internal Hub\TV series Folder\files"
mklink /J "C:\TV Series\Z" "Z:\TV Series Complete External Hub\TV series Folder\files"
This should let you have a folder structure like:
C:\TV Series
Q
<file1>
...
<file100>
Y
<file1>
...
<file100>
Z
<file1>
...
<file100>
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