Depends entirely on what you want to accomplish and the fans bought and owned.
If case fan is standard 3pin, and you buy 3pin fans, you could use a 2-way splitter and get 2 fans controlled, the remaining 2 would need either a second sys_fan header or connect direct to the psu or maybe case fan controller.
If the fan is 3pin and you buy 4pin pwm, you have 2 options. Use all 3 pwm on a 3way splitter to the header and power the existing fan from the psu or power all pwm from the psu and control only the existing fan on the header. You can't mix the fans.
If the existing fan is 4pin pwm, and you buy pwm, use a 4way powered splitter that will get power from the psu, but change speeds by pwm.
If you buy molex fans (very rare and very cheap built) it's psu only.