Can I connect 3pin fans to a 4pin cpu fan header.

You can't control a 3 pin fan. That's what the 4th pin does. Yes it'll work however it'll just run full speed (or some set % of speed) at all times

Of course. THe motherboad is a 3 or 4 pin header, not ONLY 4 pin

connecting 3 pin fans to the 4 pin cpu cooler point will not work the reason the 4 pin header is there to control speed of the fan related to temperature of the cpu why make things so complicated order some 4 pin fans you will need them is my advice mind you I could be wrong.

The motherbaord can still regulate the fan, just not quite as well as a 4 pin pwm fan.
Its not like it is bieng connected to molex to run at 100% all the time.

I know, I was being a bit dramatic. But for a CPU fan I wouldn't want it running that way. Perfectly fine for case fans. But not on top of my CPU. Putting new fans may be quieter on that cooler but a push pull config on that CPU cooler will still perform poorly. Usually a push/pull config only adds 2-5% better cooling, works even worse if the fans aren't exactly the same speed it can actually hurt performance.

Using a splitter they should be the same speed, but still a valid point.

If the OP is pushing his CPU hard, even more so if overclocking then should be spending the money the right 4 pin fans.
While you CAN use 3 pin fan and it will work, probably shouldnt for something as critical as cpu fan.

A splitter doesn't ensure the same speed. Throwing identical voltages at two fans doesn't ensure identical speeds. Bearing efficiency/blade defects/dust/wiring issues in the splitter can all cause a 100-150 rpm difference. (Is this enough to drastically cause an issue? Answer is a solid maybe. Also the pull fan has air pushed on its back the same power being applied can actually run faster with the air pushing on it.

The number of pins on a fan header are meaningless. Bit to know what you have, you have to read the MoBo Manual. On the fan side, 4-pin generally indicates PWM and 3-poin DCV. On the MoBo side, from a parts supply, inventory management, perspective, it's easier to just install 4 pin headers and use them for both 3 and 4 pin fans.

The Z87 generation of MoBos perfectly emphasized this approach. Most Z87 MoBos had all 4-pin headers, however while the CPU fans were PWM, the CHA fan headers were DCV. It appears that the "original" plan for Z87 was all PWM, as many MoBo manuals said exactly that, but at some point (after the manuals were written) manufacturers realized that this was not going to be possible for whatever reasons and put notices on their web sites.

With Z97 all but low budget MoBos have all 4 pin headers which can be used for either PWM of DCV speed control, most of them auto sensing.

The CPU does bot control fan speeds, the motherboard BIOS and / or provided software utility will.

PWMs advantage has been being able to control fan speeds down to lower rpms than DCV as when voltgae drops too low it can't "kick up" the fan from a dead stop. Recent designs have however overcome this disadvantage by applying a higher voltage at start up and then taking it down once the fan is spinning. PWM's disadvantage is low speed clicking / hum.

Phanteks came up with a design for their coolers which provided a little doodad that provided a best of both worlds scenario whereby controlled 3 pin DCV fans from a PWM signal. However, newer models have come with PWM fans, my guess being with the existence of the doodad ... it's a technical term ... PWM fan motor suppliers had to cut pricing to be competitive. The doodad not only chnaged the signal from PWM to DCV but was also a splitter to provide power to the two fans. Finally, it was able to provide low speed PWM like control to the 3 pin fans

What I would recommend for you is the Phanteks PWM to DCV (4-pin ==> 3-pn) fan hub. It splits the power and control from 1 PWM header to up to 11 fans and w/o the mess of splitter cables. You can use it w/o the supplied SATA power cable for up to 1 amp of power draw (about 6 x 140mm 1250 rpm fans) ... for more than 1 amp you will need the cable. It will take the PWM signal from the CPU header and distribute it to up to to (11) 3 pin fans.

http://phanteks.com/PH-PWHUB.html

Noctua fans were not suggested. If ya tell us where "here" is, might be able to find a suitable recommendation for your area .

That is simply incorrect. I would have agreed back in the 1990s but not here in 2017.

PWM fans are wired as follows:

1 = GND (black)
2 = +12VDC(yellow)
3 = Sense (green)
4 = Control (blue)

DCV fans are wired as follows:

1 = GND (black)
2 = +DC Voltage (red)
3 = Tachometric Signal Sensor (yellow)

PWM works by applying a constant 12V signal, but by turning the power on and off ... lengthening of OFF time to slow the fan down. The advantage of PWM *was* that you could get down to lower speeds because DCV fans at lower voltages could not start the fan from a dead stop whereas the full 12v signal from PWM could The disadvantage of PWM is a) low speed hum / clicking, b) cost and c) with some brands (i.e Corsair) , ganging too many fans on a single channel would result in loss of speed control.

The development of smart DCV fan controllers led to a best of both worlds scenario. Phanteks provided a little doodad on their coolers which converted the PWM signal to a modified DCV signal. It was capable of initiating fan start with a higher voltage and then tapering it down to low speeds, once the fan blade's inertia was overcome it could match PWMs low speed performance... and it also eliminated low speed hum. That technology was later incorporated into the Hub (PCB) which allows one to control up to 11 3-pin fans from a single PWM fan header.

When controlled in this manner, you get all the advantages of PWM fans and none of the disadvantages.

Today's motherboards (excluding very low budget models) have 4 pin headers and most will auto sense what type of fan is connected and control the fans accordingly.