You will need a 3" to 60mm (~2.36") reducer to mate up with the stock Golf downpipe. APR sells one, P/N EXH0015
https://www.goapr.com/includes/img/products/DPK0027_016.jpg
Yeah that’s what I thought. So not sure why others would be recommending a 76mm to 60mm exhaust adapter.
Didn't realize the Rev9 catback was only 2.5". Buy a used GTI downpipe and then you won't need any adapters and you'll get a little bit better exhaust flow.