Does one case fan header provide enough juice for 3 120mm fans (via splitter)?

Yes, it should, as long as when it all adds up, it's under 1v.

Its recommended to only put up to 1 amp on the motherboard fan header. So you really need to look at your fans and see how much amps they pull. For 3 fans you want to stay under .3amps per fan but I would look more at .2amps

Think you mean 1amp not 1volt

I did, thanks.

Should be okay.,

Reference:

http://www.buildcomputers.net/power-consumption-of-pc-components.html

4 (four) 120 mm fans at 6 W each = 24W ("worst case" scenerio).

Very much within the limits of what can be drawn by other components; a graphics card or DVD drive for example.

Look at the specific specifications for the fans you intend to use.

Would not purchase/use some discount molex splitter or extension cables that are flimsy and thin.

Also be sure to add up the total load that may be imposed on your PSU. Verify that the PSU wattage will be able to meet the overall power requirements.

It's not about the watts/PSU, it's about what the components are built for. Fan headers aren't built for 150 Watts like the CPU socket is. Try feeding your harddisks with 150 watts, see what happens.

GPU draws power (up to 75 watts per the standard) from the PCIE slot, and the DVD drive is usually connected directly to the PSU. The specs for what a fan header can take will vary from mobo to mobo.

To the OP, what's your motherboard? I've seen some rated as high as 3 amps, while others are as low as .5.

Interesting - so many factors coming into play.

Found this link in this forum:

http://www.tomshardware.com/answers/id-3083432/explain-fan-splitters.html

Maybe with four fans a PCB would be a neater option.... Viable?

The complete answer depends on several factors for which you have not provided enough detail. So let's look.

1. Basic "rule of thumb": as others have said, the standard fan headers on a mobo can supply up to 1.0 amps total for all the fans connected to that header. As you say, you have only one CHA_FAN header to use.
2. Fans typically consume 0.1 to 0.25 amps each. BUT fans that include LED's use significantly more. If you tell us exactly which fans you have, we can check their power consumption to be sure they fall within the 1 amp limit.
3. How to connect multiple fans to one header depends on the match (or mis-match) between header type and fan type. According the the labels in p. 19 of your mobo manual, your CHA_FAN header only operates in PWM Mode. Thus it will do a great job controlling 4-pin fans, but it cannot control the speed of any 3-pin fan plugged into it. If all your fans have 4 wires coming from each and ending in a female connector with 4 holes, you're OK. If you have 3-pin fans, then you need a different device to connect those fans to such a header. When you post your exact fan model numbers we can see what will work.
4. You say you have a "Splitter" with 4 output arms for your fans. I have not seen one of those. What I have seen with that many outputs (or more) are really fan HUBs. What's the difference? A SPLITTER has one arm with a female connector that plugs into a mobo CHA_FAN header, and two or more output arms each with male (with pins) connectors to plug into your fans. If you have a 4-pin SPLITTER, only one of those output connectors will have all 4 pins - the others will be missing Pin #3. That is all the arms a Splitter has. A HUB, on the other hand, has those two types of arms, PLUS one more that must plug into a power output (either 4-pin Molex or SATA power) from the PSU. A HUB gets power for all its fans from the PSU and thus avoids the 1 amp limit. BUT a HUB can only work with a 4-pin header using PWM Mode (that's what you have) AND only with 4-pin fans. So again, when you tell us what fans you have, we can see how that might work. Do you have a Splitter or a Hub? Tell us which make and model.

Looks like ASRock doesn't publish their chassis fan header output. They do say the CPU header has a max of 1 Amp, thus 12 watts. In the absence of any concrete information, I'd go ahead and assume the same max for the chassis fans.

Well, I can make some guesses here, based on the Sharkoon website. Their 120 mm "power" fan runs at 2400 rpm and consumes 0.19 amps. They don't list a LED fan in that series. They do have LED fans in their "Silent Eagle" line that run at 1000 rpm, and the difference between the amps for that fan and one of their 120 mm "silent" fans is about 0.12 amps. So, let's assume that your three fans each will pull at max 0.19 amps for the fan motors only, and then the one with LEDs will pull an additional 0.12 amps. That totals 0.71 amps for those three fans, well less than the 1 amp limit of the mobo header. So you should have no trouble with that.

The mobo manual specs show the CHA_FAN header with a label of "Speed Control" on Pin #4, which definitely indicates that it is putting out a PWM signal. Further in the manual in the section on configuring fan headers it does not say anything about using an option for DC Mode or Voltage Control Mode, leaving the impressions that there is no such option. So I concluded that the header can only use PWM Mode, which is incapable of controlling fan speed for a 3-pin fan. You can test this out when you have your system. Connect one of those case fans to that CHA_FAN header and observe its behaviour carefully, especially just at start-up time. What "normally" happens then is this. Immediately at start-up the fan will come on at full speed. A few seconds later as the POST process completes and a measurement of temperature becomes available, the CHA_FAN header will change its output to call for the fan to slow down a lot because the system is still cold. BUT if I am correct, your 3-pin fan will NOT slow down because the method the header uses to call for reduced speed will not work for that fan.

I suspect you have the splitter connector gender labels reversed. The connector with a line of holes is female, and plugs into the mobo CHA_FAN header. The connectors with pins are the male ones, and they are how you can plug in your fans.

I know of one good way to connect a group of 3-pin fans to a mobo header that uses only PWM Mode. You buy and install a particular Hub called the Phanteks PWM Hub, like this

https://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=phanteks+pwm+hub&N=-1&isNodeId=1

Among 4-pin fan hubs, this has a unique feature. It MUST be used with a 4-pin fan header that employs PWM Mode (your CHA_FAN header does), and it must get its power for the fans from a connection to a SATA power output from the PSU. (This avoids any concern about sufficient amps to the fans.) But its handling of the PWM function is different. Most PWM Hubs merely share the PWM signal it gets from the mobo header out to their fans, so those fans must be of the 4-pin type to be able to use that signal. The Phanteks PWM Hub instead uses that PWM signal internally to create its own group of six 3-pin fan ports operating in Voltage Control Mode, which is what is needed to control the speed of 3-pijn fans. Thus you CAN use the PWM signal from the mobo CHA_FAN header to control a group of 3-pin fans with this Hub.

If you buy and use this Hub, I suggest one change from their instructions. They say you must connect its fan header lead to the mobo's CPU_FAN header, and then plug the CPU's cooling fan into Hub Port #1. This is solely to avoid trouble for people with those odd mobo 4-pin headers that fail to provide a PWM signal. You do NOT need to do that, because your CHA_FAN header DOES provide that signal that the Phanteks PWM Hub needs. So, leave your CPU cooler plugged into the mobo CPU_FAN header. Plug the Hub's female fan lead into your mobo's CHA_FAN header. Connect its power lead to a SATA power output from the PSU. Plug all your 3-pin case ventilation fans into the Hub, making sure that one of them is plugged into Hub Port #1 (the white one). This one port of the Hub can return its fan's speed signal to the mobo header. If no fan is connected to that port of the Hub, your mobo may warn you (incorrectly) that the case ventilation fan has failed.