Installing HDD Help

I'm very noob at computers and don't want to accidentally ruin my computer.
So I want to install another HDD onto my computer, I have an SSD and a WD Black HDD already installed and want to put another 1TB WD Black HDD.

What I have is: 1.0TB SATA/64mb cache WD1003FZEX
Mother board is P8Z77-V LE ( I noticed my SSD and HDD is already connected to 2 SATA slots 6gb/s)

I'm curious it seems I have another 6gb/s SATA slot but not sure if I can use that one or should I use the other 4 3gb/s SATA slots? I have an extra 3gb/s SATA cable, should I install my HDD with 3gb/s or 6gb/s?

Also for the power wire, I noticed that the power wire already connected to my current HDD has like another plug, can I use that one or should I use a completely different wire?

If there is anything you don't understand please be patient with me, I'm very new to this. I will try to answer any questions to the best of my abilities.

Thank you