By default, FL Studio is set up to accept input from all controllers to all channels. So if you connect several controllers, all of them will play any track that is selected in the Step Sequencer.
But to prevent conflict in directions from different controllers, it is advisable to set up each controller to run on a separate channel. This cannot be done in FL Studio. It needs to be done using the software that came with your controller. It's a setting that needs to be set into the controller's memory, so it can communicate which channel it transmits on.
Now to specify which tracks in the Step Sequencer you will be playing, just right click on those tracks. The green light should go on to the right of the tracks' names, when they are selected.
Each of the selected tracks are assigned to different channels automatically. You don't need to do any set up here. The upper track in the Step Sequencer window is assigned the lowest channel number. Next selected track is assigned to the second lowest. And so on.
So, say your keyboard was set up to transmit on channel 1. It will play the highest selected track in FL Studio. Say, your guitar was set up to transmit on channel 2. It will play the second highest selected track. And say, your MIDI Controller was set up to transmit on channel 3. It will play the lowest selected track.