Thanks for Best Solution.
I made the assumption that you have 4 case ventilation fans, and that's why you are using 4 headers now. That's why I said get two splitters, each with 2 output arms. That will connect two fans each to the two headers you have that CAN control your 3-pin fans.
The connectors for 3-pin and 4-pin fan systems are all designed to plug into each other; you can easily plug a 3-pin fan (or splitter) into a 3-pin OR a 4-pin header. Likewise, you can plug a 4-pin fan into a 3-pin header. The male headers have a plastic "finger" that sticks up next to the first 3 pins, and the female connector has a groove on the side that matches that, so you can only plug them in one way. The pin layout for the first three pins is the same (with a small distinction), and the 4-pin system merely adds an extra pin for a different function that 3-pin systems don't use.
Net result is that, assuming your SYS_FAN1 and 2 headers are controlling their two fans now, when you use Splitters like I linked, each of those headers will power AND control both of the fans attached to it. You will NOT see the speed of all the fans, though. A fan header can only accept the speed signal from ONE fan, so each splitter only allows ONE of its fans to report its speed to the mobo. The other (2) fans' speeds are simply ignored.
My link was to a simple and cheap splitter design, and you don't need more. There are others with plain wires, some with three outputs, some just more expensive but don't do the job any better. 3-pin or 4-pin does not matter. Just do NOT get a HUB. Some Hubs may look very similar, but their distinguishing feature is that they all have an extra arm that MUST plug into a power output directly from the PSU. Hubs only work in 4-pin fan systems and can't do what you need.