Form 3, "Initial Statement of Beneficial Ownership of Securities", is the first filing a new insider makes. It lists the company's securities they own, directly and indirectly, including options and other derivatives.
When it is due
Within 10 days of becoming an insider: being appointed a director or officer, or crossing 10% ownership of a class of registered shares. When a company first registers its shares, as at an IPO, existing insiders file by the date the registration takes effect.
Reading it
A Form 3 is a starting point, not a transaction. It shows no purchase or sale, so Listed does not list it among insider trades. It explains holdings that later Form 4 filings change.