A combat encounter begins when the GM tells the party to “Roll Initiative!” Each hero rolls 1d20 and adds their Initiative bonus (typically their DEX). A single digit means they start combat with 1 action. 2 digits, they start combat with 2. A result of 20+ (or a natural 20) means they get all 3 actions!
Regardless of what a hero rolls on Initiative, at the end of their turn they gain all 3 actions back.