INT3 is a synonym for MEDIUMINT.

CREATE TABLE t1 (x INT3);

DESC t1;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| x     | mediumint(9) | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+