here is the commonly recommended sql command for removing posts revisions and cleaning up the wp database:

DELETE a,b,c
FROM `wp_posts` a
LEFT JOIN `wp_term_relationships` b ON (a.ID = b.object_id)
LEFT JOIN `wp_postmeta` c ON (a.ID = c.post_id)
WHERE a.post_type = 'revision';

how can i modify it to keep let’s say the last 3 revisions ?