Corsair Obsidian 750D and motherboard fan install questions

Hi

I'm going to purchase the case (in the title) soon which comes with two front 140mm and one back 140mm but i'm confused about how to install them and then an extra fan below and two fans above.

Where do i plug them all in?

I am going to put my existing 3 speed fans in the front of the case which are powered by Molex then move the two new case 140mm one up to the top while installing a new 120mm at the bottom of the case.

This is my motherboard:
https://www.scan.co.uk/products/asrock-z97-anniversary-intel-z97-s-1150-ddr3-ddr3l-pcie-30-(x16)-sata3-6gbps-intel-gbe-usb-30-hdmi-a

It has a 3 pin fan header next to the CPU fan header which has the back case fan plugged in then i have two CHA-FAN2's headers and one CHA-FAN1 header on the board which are all 4 pin headers.

I was originally going to get this:
https://www.scan.co.uk/products/akasa-pwm-fan-splitter-supports-2-pwm-fans-from-a-single-motherboard-pwm-header

So to plug the top fans together into one of the CHA-FAN 2 headers then plug the single 120mm fan at the bottom of the case into the CHA-FAN 1 at the bottom of the board as its closes to the fan.

But after researching that if your not careful with which pins and voltages you use you can blow your fan header and even the board.

Could someone please help me out here, thanks.
 
Okay so for example if i took the new 2x140mm fans out for the new cases front panel and put them on the top of the case then plugged them into the splitter which CHA-FAN* do i plug the adapter into and also (this is me presuming here) this cable splits the wattage between the fans right so they would be running at 50% as 3 pin so i have heard run at full speed if they get their full voltage.

Is there any change whatsoever i could blow my FAN header or even the board by using this splitter with the 2x140m fans that come with the case. What speed would they be running at also as my current 3 speed front fans run at low which is still rather fast as the H setting sounds like jet engine so to say.

What is your opinion on this?