USER3 is Pareidolia, The Concerned god. We seem to just be the player.
The USERs also aren't tied down to just Sensei.
We've apparently seen USER2 messing with Maya while Sensei isn't around, in 'The Scary Room' Chapter 2 Main Event:
And, it seemed like USER1 and USER3(Pareidolia) were involved with Sana, when Sensei wasn't around, in Sana's 'Ad Infinitum' Event. That's also where a Narrator seems to learn other characters see shit, and it seems to confuse itself with Sensei:
This Narrator seems to be as confused as us about what's going on, lol.
We don't seem to be a specific USER, though. Although, Sensei is apparently aware of us: