Port Splitter for EV3 motors
It depends on what you're doing, but for this purpose the Mindsensors motor multliplexer is the device you are looking for - not the port splitter.


A multiplexer (in this context) allows you to independently control/communicate with multiple devices over a single port, one at a time. A motor multiplexer will also need an external power source because a single EV3/NXT motor port cannot power multiple motors simultaneously.
The Mindsensors port splitter simply joins the 6 wires from each plug to each other plug. This is generally less useful, because all connected devices simultaneously share the same power and signals. Analog sensors will behave unpredictably, I2C sensors might work (if the device supports reassigning its I2C address, and if you keep under the EV3 port's current limits). Motors will all receive power simultaneously, none of the rotation sensors will work, and you risk burning out the EV3's motor port.