Delete revisions from the Drupal 7 node_revisions table such that only the newest 3 revisions remain for each node. It joins on the node table to make sure that the current, active vid assigned in the node table doesn't get deleted.

trim_drupal_node_revisions.sql

DELETE FROM node_revisions WHERE vid IN (
  SELECT subquery.vid FROM (
    SELECT  @row_num := IF(@prev_value=nr.nid,@row_num+1,1) AS RowNumber
       ,nr.nid
       ,nr.vid
       ,nr.timestamp
       ,@prev_value := nr.nid
    FROM node_revisions nr,
      (SELECT @row_num := 1) x,
      (SELECT @prev_value := '') y
    ORDER BY nr.nid, nr.timestamp DESC
  ) subquery LEFT JOIN node n ON n.vid = subquery.vid
  WHERE RowNumber > 3 AND n.nid IS NULL
)

Don't forget that content types with additional fields may have vids associated in other tables. They should be able to be added as a comma delimited list

DELETE FROM node_revisons, field_revision_field... WHERE vid...