You will need a BOC-3 if your trucking company, brokerage, or freight forwarding company is engaged in interstate commerce. For every state your company operates in, you will need to have a processing agent to represent you and receive documents on your behalf. The BOC-3 / Processing Agent is a document that has all of the processing agent's contact information.
Generally, the BOC-3 is valid until information on the form changes, such as the address or company name, in which case you would need to refile. BOC-3 is required in order for your USDOT to stay active and remain active.