The link mosox provided above is a good solution if you have 4-pin fans and a 4-pin mobo pinout. The splitter discussed there will draw power from the PSU via a Molex 4-pin connector but control them all (up to 3) from one PWM signal provided by the mobo pinout. You could use it two ways. One is to pug it into the CPU_FAN pinout, then connect your actual CPU fan to one of its outputs so labeled, and up to 2 case fans to other outputs. Then all three 4-pin fans are controlled by the system that focuses on CPU cooling. Alternatively you can run your CPU fan off its intended mobo pinout alone, and use the splitter connected to a 4-pin SYS_FAN pinout on your mobo. Then all of your case fans will be controlled by the case fan cooling control system, and you can plug up to 3 fans into the splitter (including one into its output supposedly labeled for the CPU fan, which you already have covered separately).
However, if you are trying to control 3-pin fans, you cannot use this splitter. It only works for 4-pin fans because it requires use of the PWM signal for speed control. For 3-pin fans you can connect two such fans together in parallel off one mobo pinout, but probably not more than that per pinout. The limit comes in the starting current required for 2 or more fans in parallel off one supply line. To do this you have two choices. One is to wire the two fans together from one connector. Splice together the black (Ground) wires and the red (+ 12VDC varying) wires from the two fans, but do NOT splice together the Yellow wires. Connect only ONE yellow wire to the connector, and leave the yellow from the other fan not connected to anything. The other choice is to buy a splitter that feeds two 3-pin fans from one connector, BUT watch out for these. The ones I have seen connect all three wire sets together and fail to leave one yellow wire disconnected. If you get one of those, snip off ONE of the yellow wires.
Why only one yellow wire? Those wires on each fan send a pulse signal (2 pulses per revolution) from the fan motor to the mobo where it is monitored by a counting circuit to figure out actual fan speed. If you connect two fans' yellow wires to that counter, it gets two pulse signals superimposed on each other (so twice as many pulses to count), and the timing of them keeps changing (because the two fans never run at exactly the same speed). The fan speed circuit will never figure this out and you will get ridiculous readings. It can even result in warning messages that the fan has stopped working! Connect only one yellow wire and you get to see that fan's speed correctly, while the other fan's speed will never be known.
Of course you have the option to power one or more fans directly from the Molex 4-pin power connector from the PSU, which runs that (those) fan(s) at full speed all the time. Such a system does not monitor or display the fan speeds at all. If your case fan(s) do not have connectors on their wire ends for Molex 4-pin outputs, buy simple adapters for that. Alternatively you can buy 3rd party fan controllers that mount in the front of your case (usually in a 5¼" device slot) to give you manual control of several fans. These units typically draw power from one Molex 4-pin and have separately controlled output connectors for several case fans. May also have ways to measure and display actual temperatures inside your case with sensors supplied with the unit.