Finally UNIX_TIMESTAMP worked and my column of type timestamp(3) can be cast to string

select myts_col,UNIX_TIMESTAMP(CAST(myts_col AS STRING)), CAST(myts_col AS STRING),DATE_FORMAT(myts_col,'yyyy-MM-dd HH:mm:ss') from mytable

mycol_ts    conv_col
2024-11-06 06:34:53.316 1730874893