Yep.they can be replaced. Check eBay or I found this link. I've never used them though. Budget £20-40 for replacement
https://www.wingmirrorman.co.uk/wing-mirror-indicators/audi-wing-mirror-indicators/audi-a3-wing-mirror-indicators/audi-a3-2012-2020-wing-mirror-indicators/audi-a3-2012-2016-wing-mirror-indicators/audi-a3-hatchback-2012-2016-wing-mirror-indicators