I've not used a KPC3 but there are typically three things that affect a TNC's ability to "make good data".
- the volume of the audio from the radio to the TNC
- the modulation of the audio from the radio to the TNC
- the amount of background noise in the audio
All of the above is optimized 'by ear'. Prior to connecting the radio to the TNC, you can listen to the audio to evaluate both the volume and the background noise. A good antenna is part of getting a good solid signal and thus a good solid sound.
Then, it depends on what level of audio signal is expected by the TNC. Some configurations need a speaker level audio signal while others need a line-level signal. The old "attenuated patch cord" was used to go from speaker to line level but this still required getting the volume correct for optimal data.
Finally, the squelch on the radio comes into play. Too much squelch and the audio gets clipped. I've seen a number of configurations of 'receive only' iGates which called for the squelch to be wide open.
All of the above is just to get good data. After that, it is up to your iGate software to get the data to the Internet APRS site.