The KXFL3 is actually 4 different filters that are shared between Phone and CW modes. Containing 2 filters for each mode, the filters switch points are:
This means that you get the value of 4 filters in other radios for less than the price of one in other radios!
A: Yes. As with all Elecraft options, you can install any option at any time
A: No. Elecraft techs will take care of calibrating your KX3 with the KXFL3 installed as it is built for you.
A: There is a calibration procedure you'll be following when you install the KXFL3 but the test signals you'll need to conduct it are available inside the KX3. Calibration instructions can be found in the KX3 Owner's Manual and the KXFL3 Installation and Calibration Instructions.