TL;DR: you won't be able to achieve this now.
Long explanation is that QA_MYSQL_USERS_PREFIX just changes the prefix of the ^users table. So you can change it whenever you want. The problem is that if you already have users in a table and you stop using it... well... users will be gone.
You might think about migrating them but that's not as simple as it sounds. If you re-insert the users, then the user IDs will be different now. If you just force the user IDs to be the same, you will realize that many of them will be triplicated (e.g.: user ID 1, which is usually the super admin).
So you should first update all single user IDs to a user ID that is unique to all three sites and also update all references to it, and this is not something with the complexity of just adding/removing a prefix. Then reinsert all records keeping the original ID. So I'd recommend to leave things the way they are now unless you know what you're doing.