What is an SEC Form 3?

Form 3 is an insider's first report of ownership: what they own in a company when they become an officer, director or owner of more than 10% of its stock. It's due within 10 days, and later changes go on Form 4.

A Form 3 has no trades in it, only holdings, so it tells you where an insider started. When a company goes public, its insiders file Form 3 by the day its registration takes effect.