Sei que ficou claro, mas para tirar qualquer dúvida.
Does the attack granted by the third benefit of the Sentinel feat take place before or after the triggering attack?
The bonus attack takes place after the triggering attack. Here’s why: the feat doesn’t specify the bonus attack’s timing, and when a reaction has no timing specified, the reaction occurs after its trigger finishes (DMG, 252). In contrast, an opportunity attack specifically takes place before its trigger finishes—that is, right before the target creature leaves your reach (PH, 195).