Based on the wiki, no — Michael Penix Jr. is not expected to start in Week 3. He's been recovering from an injury dating back to the start of the season, and both he and Tua Tagovailoa were dealing with health issues heading into the Thursday night game against the Saints. Neither QB was expected to be available, pushing their potential return timeline toward Week 4 (from the Michael Penix Jr. and Atlanta Falcons pages).
So Week 3 looks like a Cooper Rush or third-string situation for Atlanta.