Timestamp(3) means to represent 3 digits of millisecond value of a timestamp. Example for this would be - 2017-07-07 13:28:48.840. In this 840 represents the millisecond value. You can use this timestamp value for your Insert statement purposes.
On the same note, timestamp(6) represents 6 digits of millisecond value of a timestamp. The above value incase of timestamp(6) might be 2017-07-07 13:28:48.840003. Its basically the representation of a timestamp at different granularities.