First, relax and find your 4 x PCE-I ports
To begin with, the plugs you want are named 6/8 PCE-I for short because you can make either a 6 or 8 prong plug.
In baby terms, it's a plug of 8 blocks but divided 6/2 and looks like this:
|_|_|_| --> |_|
|_|_|_| --> |_|
The --> being small wires tying up both parts.
Essentially, you can make a 6 port and an 8 port adapter with the same plus from the PSU.
The S3 Uses only the 6 box plug, let the other 2 box plug roam free or cover with electrical tape: I leave mine roaming.
There are 4 PLUGS from the PSU to plug into the S3, NOT 4 WIRES on the PSU; Each port should be attached to a clone of itself.
Each PCE-I WIRE should have two 6/8 PCE-I PLUGS.
If you have a non-modular, pick a plug and seclude it, look at the tip and see if it's what you need, rinse and repeat until you found both wires.
For plugging in the S3, you may need to try and open the clamp on your PSU wires to fit it in. ( I know S3 Ports are a bit too tight )
I know it's fucked up on the PSU, I know you expect a 6 box plug to fit in an 8 box adapter, not a 6 + 2 plug with a clone of itself attached to it.
I know it sounds ridiculous but trust me, it's one wire with a pair of PCE-I ports at the end.
That's why I get Modular PSU's, lot less pain in my rear end; just pick out the wires you need slap on a clip and off you go.
This means if you find one PCE-I 6/8 plug, its twin is attached to it; find both branches and you're almost done.
They will have one end that has 8 prongs (if modular) and the other end has 6 prongs and likely a smaller 2 prong to adapt an 8 prong adapter.
If you have a Modular, this is quite easy. Simply rummage through the pile of wires they give you and only use the ones you have.
Set each wire you need aside and put back wires you don't need back in the box.
Note that a fully modular PSU has one extra tail you need to find to put in your jumper wire.
If you have a non-Modular as in there are absolutely no detachable wires, you can use something to mark the wires you need as you rummage.
Start at the root, at the PSU and work your way out.
Once you got the wires you need, time to start plugging them in.
PS: 750 watts is overkill for one S3 and not enough for a pair OC'd, go for the 650 for solo operation - I use those easily but you need 4 x 6/8 PCE-I ports
I use a 650Watt Corsair and Seasonic Modular on 3 S3's, great choice
PM me if you need more help, I spent a lot of time understanding PSU's and guides on this forum get quickly outdated but serve to give an idea.
I know it's weird, especially if you didn't get Modular PSU... I don't regularly buy Corsairs so I don't know if the CX are Modular or not.
That model line should be modular I believe, if so, one end of the wires you want will have 8 blocks on one end.
You may also have a non-modular part to your PSU (Semi Modular), if so, there should be at least one wire you want in there.
Took me a while to figure out the non-modular aspect, seems like you never have the right wires and of course impossible to know every PSU's wire configuration.
Hope this helps.
-GigaBit
