There are two solutions available.
One is, you buy an adapter that allows you to plug this 3-pin fan directly into a 4-pin Molex output connector from the PSU. This will make that fan run at full speed all the time, AND there will be no way to read its speed.
Second option is to buy a 3-pin fan splitter adapter that lets you plug two fans into one mobo port. Then you can plug this fan plus one you already have connected into the SYS_FAN port already in use. Or, you could connect two of your existing fans together on one port, freeing up a port for use with this large fan. Either way the fans all become speed-controlled, with speed readout possible for the fans. (Well, actually, on the port with the adapter and two fans, only one of...