Most of this depends on exactly where your fans are plugged in. But another important factor is that the case came with TWO older-design 3-pin fans for the top rear and rear panel positions. From your post I suspect you have only ONE of those installed on the back panel. Is that right?
That case also includes a small device like a circuit board or box they call a Smart Device V2. See the lower right part of Page 1 of the case manual. This is what most call a fan HUB and it is used to connect several fans to a single mobo fan header for control of all of them together. HOWEVER, such a device can work ONLY for the new 4-pin PWM fan design. It can NOT control the speed of the 3-pin fans that came with the case.
I will assume that you have all three Corsair LL120's (these ARE 4-pin PWM fans, but with lights, too, I believe) installed, two in the front panel as intakes and one in the top rear position as an exhaust. Then you have ONE of the NZXT 3-pin fans supplied with that case mounted in the rear panel as a second exhaust. If that is wrong, post corrections back here.
Before making any changes you need to understand a bit about how fans are controlled and where they are connected.
First point: these days many fans come with lights in their frames. Such units really are TWO devices in one unit: a fan MOTOR, and the LED lights in the frame. These are separate, so they each have their power power cable with different connectors on the ends that go to different mobo headers. The cable for the MOTOR ends in a female connector (with 3 or 4 holes) about 3/8" to ½" wide that has two ridges down one side. It goes to a mobo male header with 3 or 4 pins, and right beside the first 3 pins of that is a plastic tongue sticking up. The female connector ridges fit beside the male header tongue so there is only one way to plug it in. The cable for the LIGHTS ends in a wider wider connector that may have four evenly-spaced holes, or 3 holes looking like had four but one of them is blocked off. This goes to a different mobo male (with pins) header. You have not asked for help with lights, so I won't deal with them now unless you ask later. I'll only talk about the motors.
There are two basic types of fan MOTORS. The older style uses a cable with 3 wires (Black, Red, Yellow) and a connector with only 3 holes. Its speed can be controlled by the header ONLY by changing the Voltage supplied on the Red wire at Pin #2. The Smart Device V2 cannot do this, so there's another way. That is what came with your case.
The newer fan design has 4 wires and pins (holes) (wire colour codes are different), and that is the LL120's. Their speeds CAN be controlled using the Smart Device V2. That device actually comes with three output sockets, so it would suit your LL120's.
So here is how you can set these up. The single 3-pin fan at the rear should be plugged into one of your mobo's CHA_FAN headers. See your mobo manual, p. 1-14. The closest CHA_FAN header is No. 1 near the mobo rear, mid-height. On p. 1-14 it is Key No. E. Do NOT try to use the adjacent AIO_PUMP Header, Key No D. I'll talk later about configuring this header.
Now find the Smart Device V2. It has a couple of cables coming out if it. One ends in a wide connector to plug into a SATA power output connector from the PSU. The other ends in a 4-hole female connector to plug into a mobo CHA_FAN header. For this I suggest the CHA_FAN3 header (Key No. B) at the mobo front edge near the top. The case kit should have come with a three-output cable for plugging in your three LL120 fan cables. Then the INPUT connector of this unit plugs into one of the Smart Device V2's three output sockets. Now you have all three of those Corsair fans' motors connected to the Smart Device V2, and it has power from the PSU and a control signal from the CHA_FAN3 header.
Once those are connected up, boot up into BIOS Setup immediately. Navigate to the Q-Fan Control screen - see manual p. 3-7 .Go first to CHA_FAN3. At upper right set this to PWM Mode. At bottom set to Standard Profile. Next, go to the CHA_FAN1 header where your rear NZXT 3-pin fan is plugged in. Same Standard Profile at the bottom, BUT at upper right set this to DC. Use the Esc key back to Main Menu (p. 3-3), then the F10 key to get to the Exit Menu (p. 3-20. There choose Save Changes and Reset to save the settings and reboot.
This will put all four of your fans under automatic control of their speeds according to the pre-set "fan curve" and guided by a temperture sensor on your mobo. CHA_FAN1 will send to the NZXT fan at the rear a set of DC Mode signals to control it. CHA_FAN3 will send to the Smart Device V2 a set of PWM signals to control all three LL120 fans plugged into that device.
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