I’m wondering how hard would it be to have 3 sites running at the same time. 2 sites would be archives and read-only, and one site would be the one with new discussion.
How hard would this be? Would it be doable with the lowest or second lowest price Digital Ocean droplet?
Since additional multisite instances require almost no resources at all, I don’t see why this shouldn’t work. I’d recommend you try it – upgrading to a larger instance is pretty painless if it turns out you need it.
Multisite installations are a bit tricky, though, so you should plan some time for careful study of the instructions and some experimentation 
OH?! Is that the case?! Wow. Thanks for letting me know!