Concatenate two column with another text and insert in third column.

+-----+------+----------+-------------+
| id  | abbr | trans_id | forth_field |
+-----+------+----------+-------------+
| 10  | mm   | mm0010   |             |
+-----+------+----------+-------------+

UPDATE table
SET forth_field = CONCAT(abbr, '00', id)

+-----+------+----------+-------------+
| id  | abbr | trans_id | forth_field |
+-----+------+----------+-------------+
| 10  | mm   | mm0010   | mm0010      |
+-----+------+----------+-------------+