Need Help Managing My Hard Drives

Hello Everyone. First i am inexperienced with Linux and Cent-OS. So please forgive my ignorance. 

I will try my best to explain my problem and what i actuaclly want.

I have a dedicated server with 3 Hard Drives and 1 SSD. The Centos 7 is installed on the 80 GB SSD and now three hard drives are free.

Among these three hard drives two are on raid controller. Which logically means i have two extra hard drives, (As raid will show two same hard drives as one).

Now i have two hard drives free which i want to use to store my data. But the problem is that, previously i was storing my data in /var/www/html

And now if i start uploading my files to /var/www/html the SSD will get full which i dont want. Even if i bind one of my hard drives to that directory the SSD will get full.

So Here IS The Question.

1. How Can I Use Both Of My These Extra hard Drives to work on a same directory. I made a previous thread about this and one of the mod also replied thanks to him, but i messed up and had to reinstall OS.

2. After making these two hard drives work together, where should i point them? so that my SSD dosent gets full. 

Or in simple words, I want my two extra hard drives work together and i upload files to them and users access them through Nginx web server but i dont want to upload files to my SSD.

I hope i made clear what i want. And please if you have solution type all of the commands because i am inexperienced.