DROP TABLE IF EXISTS post_images;
DROP SEQUENCE IF EXISTS post_images_id_seq;

CREATE SEQUENCE post_images_id_seq;

CREATE TABLE post_images (
  id BIGINT NOT NULL DEFAULT NEXTVAL('post_images_id_seq'::regclass),
  post_id BIGINT,
  image_url VARCHAR,
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (id)
);

INSERT INTO post_images (post_id, image_url, created_at) VALUES
(1, 'images.com/1', '2021-03-12'),
(2, 'images.com/2', '2021-03-15'),
(2, 'images.com/3', '2021-03-20'),
(3, 'images.com/4', '2021-04-13'),
(1, 'images.com/5', '2021-04-15'),
(3, 'images.com/6', '2021-04-17'),
(3, 'images.com/7', '2021-04-18'),
(1, 'images.com/8', '2021-05-01'),
(1, 'images.com/9', '2021-05-02'),
(3, 'images.com/0', '2021-05-13');

SELECT
  id, post_id, image_url, created_at, updated_at

FROM (
  SELECT
    post_images.*,
    ROW_NUMBER() OVER (
      PARTITION BY post_id ORDER BY created_at DESC
    ) AS rn

  FROM
    post_images
) post_images

WHERE rn <= 3;