Thank you mjolnarn for your input! I'll play more with the Aunsoft converter program to see whether I might be lucky enough to find any workaround. Thanks again.

I know my Avid MC 3 is too old to edit the AVCHD MTS files directly. So I found a video converter program called Aunsoft MTS/M2TS Converter to help convert my MTS files to the Avid DNxHD (*.mov) format and then imported into MC for editing. Unfortunately, the converted DNxHD MOV file still could not be supported by my old MC :-( Only the MC 5/6 can