DBA for US sole proprietor living abroad
Do I need to fill a DBA? and if so, in which state since I haven't resided in the US for almost three years?
From your question I understand that you never formed a legal entity for this business. DBA stands for "doing business as" and it acts as an alias to existing entity, be it a separate legal entity such as LLC or corporation, or yourself as a sole proprietor.
Assuming you run your business as a sole proprietor you can register a DBA. In most states sole proprietors would register a DBA in the county where their business is located. Some states (like Ohio or Oklahoma) allow you to register the DBA with the Secretary of State.
In my opinion for business such as yours and given the fact that you don't reside in the States for so long you should simply form an LLC and run your business under that LLC. You can pick one of the incorporation-friendly states such as Wyoming or Delaware.
I hope that helps.