The simple answer to this is of course the scoring of three goals. However, when Stanley Shoveller scored eight goals against Belgium at the Antwerp Olympics in 1920, how many hat tricks was it?
Some might say two and two thirds hat tricks. However, others argue that it is in fact six hat tricks. The rationale for this is that the first three goals represent the first hat trick and every goal thereafter creates a new hat trick. So, the first three goals plus five more makes six hat tricks, unless you know different?!
