Greetings!
Chiming in a little late, but it looks like there is only one case style for your computer and it is setup to have one 3.5" drive in the bottom and two 2.5" drives can be installed on the inner front of the case. There are sufficient connectors on the motherboard for all three drives (a fourth connection is for your optical drive).
https://topics-cdn.dell.com/pdf/inspiron-3670-desktop_service-manual_en-us.pdf
Scroll down to section 29 "Replacing the 2.5-inch hard drive" and it will give you the info.
Also, it appears that if the m.2 slot on your motherboard is not already used by an optane card and is free, you can install the 1TB SSD you were asking about in another post. It looks like it is the M type (NVMe) and...