The Immigration requirement is 2 months seasoning of your funds. The reason you show 3 months of bank statements is to ensure that you have at least 2 months of trasactions shown. A "2 month statement" today would show August (up to the 20th) and all of July so not 2 full months. All of June and July plus part of August ensures two full months for Immigration purposes hence the 3 months statement.
Easier and better to exit then start again on a Non-O as extending a Non-OA means you have to meet two requirements every year, the mandatory Thai insurance and the financial requirements such as 800k seasoned.