Yeah, hat's another way to handle 3-pin fans. It is basically a manual two-speed switch for a bunch of 3-pin fans, with power drawn from the PSU directly.
What I had in mind for OP was a unique fan Hub device by Phanteks. It is different because it uses the PWM signal from a mobo header using PWM Mode to create its own group of 3-pin fan output ports that use Voltage Control Mode. Since 4-pin fans actually can be controlled this way (although not technically ideal), that Hub can deal with any mix of 3- and 4-pin fans, as long as its host header is using PWM Mode. Like other Hubs, it gets all power for its fans from the PSU and does not load up the mobo header. But OP would need hat ONLY if he / she has 3-pin fans to control.