MariaDB 10.3

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CREATE TABLE tablename(timer DATETIME)
INSERT INTO tablename(timer) VALUES
('2021-04-13 15:31:59'),
('2021-04-13 15:29:59'),
('2021-04-12 15:31:59'),
('2021-04-12 15:29:59'),
('2021-04-10 15:31:59'),
('2021-04-10 15:29:59'),
('2021-04-08 15:31:59'),
('2021-04-08 15:29:59');
SELECT *
FROM (
  SELECT *, DENSE_RANK() OVER (ORDER BY DATE(timer) DESC) rnk
  FROM tablename
) t
WHERE rnk <= 3


timer
rnk


2021-04-13 15:31:59
1

2021-04-13 15:29:59
1

2021-04-12 15:29:59
2

2021-04-12 15:31:59
2

2021-04-10 15:31:59
3

2021-04-10 15:29:59
3

timer	rnk
2021-04-13 15:31:59	1
2021-04-13 15:29:59	1
2021-04-12 15:29:59	2
2021-04-12 15:31:59	2
2021-04-10 15:31:59	3
2021-04-10 15:29:59	3