A document that must be filed with the Securities and Exchange Commission (SEC) by an insider affiliated with a public company's operation or by any investor owning 10% or more of the company's outstanding shares. Taobiz explains Form 3 This document must be filed with the SEC no later than 10 days after an insider becomes affiliated with a company, and it must be filed for each company in which a person is an insider, regardless of whether or not the insider has an equity position in the company at that time.