Any 3-pin fan plugged into a true 4-pin port will always run at full speed.
There is one way this MIGHT be changed. SOME mobos allow you to change in BIOS Setup the way that some of their 4-pin SYS_FAN ports work. So look in BIOS Setup for those ports' configuration settings. There may be a place for you to choose to make a 4-pin SYS_FAN port operate EITHER in 4-pin (PWM Control) Mode, OR in 3-pin (Voltage Control) Mode. If you can, set it to the 3-pin Voltage Control Mode. It is common these days to see those options available for CPU_FAN ports, but only on SOME mobo's SYS_FAN ports.
If you don't have that option, your choices are either to replace your 3-pin fans with 4-pin models or buy and install a manual third-party fan controller module, which might be more expensive. Another more limited option is that SOME 3-pin fans come with little "low noise" adapters that you insert into the fan's power leads. These are actually little resistors that reduce the fan's speed to something slower, with less noise and less cooling. BUT this yields a fixed speed, not automatic control.
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