Different attachment systems as far as I'm aware so they aren't interchangeable. Google aug a2 receiver for some pics. The a2 has a portion of the reciever that the rail or optic looks to just slide on and lock in place. VS the aug a3 and a3 m1 its more you take off a thing that's more solidly attached to the receiver in a different way (looks like it goes in a slot maybe?). But it's clear to the naked eye the methods of attachment are different enough that off the bat they wouldn't be interchangeable. Theoretically maybe someone could come up with some widget that fits into the a3 that gives it that a2 attachment but given the extremely low market for that I don't think any commercial part company seeking to make a profit would look into it because there aren't that many AUG's in the wild even less when you shrink down to a specific model and I doubt there are that many aug a2 scope assemblies floating around either since the base model is 20 years old and it never had a real chance for mass civilian acceptance in the US because of Clinton and the AWB at the time.
If I'm wrong I'm sure malenite will be along shortly to correct me.
Edit: in the time it took me to write this on my phone it was answered already.