record.copy()

returns a record object. there isn't an internal id associated with it until you

save()

the resulting object. something like:

var objRecord = record.copy({
        type: 'customrecord_sc_s3_files',
        id: sThreeRecordId,
        isDynamic: true,
});
objRecord.setValue({fieldId: custrecord_sc_s3_file_link, value: lowResString});

var newInternalId = objRecord.save();  //here's your new internal ID