Welcome to the forum.
That configuration will work fine. You have split the I/O load over your three physical HDD's. For video editing, your performance should be about as good as it gets, without going up to RAID's for the media, and for the Exports. Even then, I am not sure that you would get the BFTB (bang for the bucks).
The one consideration is when one comes to "housekeeping," after the Project is done. Still, if you set up the folder structure for you Projects, and by your Projects, it will not be hard.
I would create a folder on your HDD's, for each Project. Then do just as you are doing, but the various Scratch Disks would be located below that Project folder. This will allow you easily find that Project by its name, and just Delete that "root folder," removing all of the sub-folders with the Scratch Disk folders included. This takes but a moment, when one is about to start a Project. Just create a folder [Project Name] on each of your drives. Then when you start a New Project [Project Name], set the Scratch Disks, as you have listed, and locate them under [Project Name] on each of your HDD's. Done.
I take a slightly different tack, but with reason, and at the cost of some performance. As I migrate many Projects between my workstation and my laptop, I will use my FW-800 externals. There, I will set up my one Project folder, [Project Name], and below that, all of my Assets sub-folder, plus all of my Scratch Disks. The Assets sub-folders will be filled with Copies of my Assets, gathered from my NAS, or from other areas on my system. I try to never work with original Assets. This costs me ultimate performance, such as you will have, but everything is then self-contained, so I have everything that I need, all in one place, for when I move the Project around. When finished, I have one root folder, [Project Name], to remove, and that takes all sub-folders with it. Of course, I always archive the Project first, and onto another external HDD for safe-keeping, in case I ever need to revisit that Project, and need to re-edit. Were it not for the need to migrate Projects, I would use exactly the setup that you have.
Job well-done, and am so glad to hear the report, when one has at least 3 physical HDD's!
I would just consider the Project setup, mentioned above, to make cleanup easiest on you. All it takes is a moment of planning and setup, and you have all bases covered.
Good luck, and happy editing,