Good question. Scorefitter dates back to Classic Studio and Volume 3 was an addition purchace then.
I have every version of NGStudio Ultimate going back to PS16 installed. I did that in sequence. I don't have Scorefitter volume 3 in my list of installed programmes, but I do have Volumes 1-2 installed. So I looked at PS16 and it has 9 catagories of scorefitter songs, including World.
Now, if you look at this link:
https://support.corel.com/hc/en-us/articles/217124327-How-to-tell-if-Scorefitter-Volumes-are-properly-installed-in-Studio
you will see that Volume 3 adds songs to the the World catagory to the 8 from volumes 1 and 2. So every version of NG has all the scorefitter catagories, so I suggest you check your version 21.5, and it does have all the songs then you don't need to worry.
I'll have to install one of my Standard or Plus builds to see, but from memory Standard doesn't have so many songs, and what I think they have done is rolled vol 3 into 1 and 2 for NGStudio and then let the number of songs shown be dictated by your serial number.
ADDED - hang on, there are some songs listed in the World catagory for volume 3 that don't show up in NG STudio.... I have 8 catagories, so I DON'T have all the World songs in PS 16-24 but I do have more than are listed for Vols 1-2. Do you have them in 19?
That support list must refer to Classic Studio.
I think I've got a Vol 3 disc somewhere....