The only way in which to remove data from your database, would be to remove data. In other words, indeed delete. The reality is, your database requirements are over that which you are trying to get down to. 3gb is quite a small size to be trying to reduce your database size to. Looking at your license, your site is not a new site, so there is quite a substantial amount of data, and to be honest, 6.5gb isnt very large