Correct. Your primary hard drive would have all your installed programs, on the FAST drive. Then you put your data on your secondary drive......things like photos, movies, kids stuff, school assignments, whatever it is that you do. Then you simply copy that to the 3rd hard drive.
The reason for this is your primary FAST drive stays for programs only, to keep things lean and fast. Then you keep your data on your 2nd drive. BUT if it craps out at some point, then you're screwed and you lost your irreplaceable data. BUT if you have 2 drives instead of one, then you have a duplicate set of data, so if one data drive goes down you're fine because you have another one ready to go.
It all depends on how paranoid you are about data security, and if you have data that can't be replaced. Myself, I have stuff that is absolutely irreplaceable and if I lose it, I'm finished. So I have 2 copies of all my data. I'd rather spend the $100 for an extra drive than cry when my one hard drive fails and I lose EVERYTHING.
Just food for thought, in case you have important data. You know best what you'll use this system for. So just consider it.......packing all your eggs into one basket might not be the best couse of action for you, but it's your call. Just wanted to throw out the suggestion in case.