I think it would be pretty easy to sort in your 1+2+3 case. Let users natively ban instances.
Say you hate instance 3. If you go into the 1+2+3 thread, you will only be able to see or interact with 1+2 comments, as all others wont even load for you.
The tricky part is if 3 users can see or interact with your comments. It might be the case that you literally just wont see anything downthread if one of them replies to you.