External footswitches plugged into the pedal jacks on the FM3 can be used in a number of ways:
- You can refer to them directly in any modifiable parameter directly - for example, you could assign a switch to turn a chorus on/off by attaching it to the effect bypass in the chorus block
- You can assign them to be one of the External controllers - External 3 for example. Then you can use External 3 to turn the chorus on/off rather than referencing the pedal directly
- You can configure them to work as a Stand-In switch. Stand-in switches are configured to refer to a configured switch setting in a layout. So the footswitch will do the same thing as if you had loaded the layout on the FM3 and pressed the switch.
For example, using your 4 button footswitch, configure Layout 8 switches 9, 10, 11, 12 for the functions you would like to assign to your switch. Using letters for your 4 button footswitch, you could assign A to Layout 8 Switch 9, B to L8-10, C to L8-11, and D to L8-12. You cannot change what your 4 button footswitch is mapped to on the fly - it will be fixed to refer to what you configured in Layout 8.
With a simple external footswitch, you cannot have any LED reflecting the on/off state because there is no way for the FM3 to tell the footswitch whether or not to show an LED.
If you go with a MIDI controller, you can get one that operates as a generic MIDI controller or you can get one that understands the Fractal 3rd Party MIDI protocol.
With a generic MIDI controller, you can change presets via program change messages and you can use CC messages to do anything that is in the MIDI/Remote pages. There are a number of things that the FM3/FC can do that are not exposed via MIDI CC messages so make sure you understand what you are able to do. As with a simple external switch, there is no way for a generic MIDI controller to accurately reflect on/off states of effects, channels, etc.
With a controller that understands the Fractal 3rd Party MIDI protocol, you should be able to do anything that is documented here:
https://forum.fractalaudio.com/threads/third-party-midi-spec.140602/