The PS scratch disk defaults to your system root drive as do config and presets to your user document folder. No need to do anything if you don't want to change it. The rest is more or less a hypothetical. SSDs these days are robust enough these days, so having a dedicated third SSD for scratch would not yield any benefits. I'd only get worried if the drives get too full and you're back to that old fragmentation problem with cell-based storage devices that can cost tons of performance and ruin cells with too many write operations. Depending on how you work, that may be a long time away, though.
I am using my boot drive for scratch at the moment. I do have a 1Tb 970EVO, but the boot drive is one of two 960PRO nvme drives I have, and happens to have the most free space. I have 64Gb RAM but see some surprisingly large Photoshop Temp files sometimes. Right now I have 10 mediums sized single layer JPGs open, and nothing in the scratch space
Bugger!!! I actually noticed the orphaned Photoshop temp files about a week ago, on the other 960PRO drive that I used to use as scratch space. (You can see the date!) I was answering another thread about scratch space and then forgot to delete them. My bad, but useful to demonstrate 1) how nearly 20% of that drive's space is being wasted on a trapped file, and 2) that it pays to check now and again. Any time Photoshop does not shut down properly (i.e.it crashes) any temp files will be left trapped (orphaned) on your scratch drive.