Now i'm running with 2 3tb wd purple and i want to add 1 more (or black) but i don't have the space in my case to put it in the third hdd
I'm curious about the temps if i put a third card to a non well cooled area
The drives are rated for operating temps up to 65C. I doubt they will get that hot down there, the PSU shouldn't affect them too much and they are right up at the front, which should be the coolest part.
Either way, you could DIY something for sure if you need to, before that is NEEDED I wouldn't worry too much about it.