Connect two of your devices to the first mixer by running cable from the device's outports to two of the mixer's in-channels.
Connect the first mixer to the second mixer by running a cable from the first mixer's out-channel to an input channel on the second mixer.
Hook up your third device by connecting cable to its out-channel and running it to an input on the second mixer.
Connect your second mixer to the auxiliary input channel of your amplifier and a set of headphones to both mixers.
Turn your equipment on and check that it's working by playing music on each device and using the cross-fader on both mixers to ensure that everything's connected properly.
Start to experiment. You now have three devices to play within your mix and can apply additional effects to to the output of your first mixer through your second. The first mixer now acts as a slave as its output is routed through one of the second's input channels. You can now mix tracks on the two devices connected to the first mixer, and bring in a third by cutting or fading in from the second.