Hi, so it seems. Would this configuration be OK?:
OS+Software 500Gbs NVME
Project + Source files: 1 T SSD (what specs should I seek?)
Scratch + Media files: 2T HDD (3.5" size is the only room I can handle in my case)
If you are editing audio/video files, then the scratch space only needs to be as big as your largest expected project temp files. I would also add as much memory as physically possible to avoid using the Windows pagefile.sys and swapfile.sys.
Depending on program you can specify which is your scratch drive for temporary files. Some programs will automatically use windows TEMP path or USER TEMP PATH system variables without asking. You'll need to consult your user documentation for the program in question to see what is used, or consult with the product forums as to how scratch files are handled (located) If you are lucky, you'll be able to load your projects off of some large DAS/NAS and then run the scratch file locally on some RAID array with MLC cache for the scratch work.
This is a bit of research you have to do on your own. Sorry I can't be of more help. I don't want to give specific advice without knowing the programs you use and having had personal experience with them.
*interesting sidenote: This is one reason AMD created a video card specifically dedicated for video editing. It attached fast SSD caching DIRECTLY to the video card so editing become a ton easier with large files. It removed a lot of bottlenecks found on common systems.