INSERT INTO tbl3 (username, password, email, ip)
SELECT name, pass, email, null FROM tbl1
INSERT INTO tbl3 (username, password, email, ip)
SELECT user, password, email, ip FROM tbl2
INSERT IGNORE INTO `base_3` (
SELECT
`id` AS `id`,
`name` AS `username`,
`pass` AS `password`,
`email` AS `email`,
NULL AS `ip`,
NULL AS `db`
FROM `base_1`
UNION
SELECT
`id` AS `id`,
`user` AS `username`,
`password` AS `password`,
`email` AS `email`,
`ip` AS `ip`,
NULL AS `db`
FROM `base_2`
)