When my phone MOTOROLA G3(2015) is updated to Marshmallow, it's not showing files when connected to PC. While connecting, it shows options as charging only, PTP, MTP, MIDI. When I select MTP, it shows the external card, but not showing the contents, where as internal memory is detected and all files are shown.
Settings -> Apps -> Show system (three vertical dots) -> External Storage -> Storage -> Clear data and Clear Cache
Settings -> Apps -> Show system (three vertical dots) -> Media Storage -> Storage -> Clear data and Clear Cache
Reboot your device and connect device as MTP and now you can access files in Internal as well as External SD card from your PC.
Verified in Moto G2(2014) with Android Marshmallow and should work in Moto G3 too. If you are still unable to access the files, you can clear cache partition and reboot your device and check it.