Since it's been more than a year since I wrote the OP, I thought i would give a little update.
Basically, nothing's happened.
I mentioned that of my 3 unbelieving friends, I had only heard from one of them. That's remained true in the 12 months since then. I actually didn't hear so much from that one friend this year though, just a couple of times. Though I stopped visiting the site I knew her from and left her a way to contact me, if she ever decides to. As for the other 2, they've been just as gravely silent as they were when I first wrote this topic. One of them I never saw sign in since spring of '19. The other, presumably, still holds a grudge against me for arguments from years ago.
I had prayed often this year to hear word from them again to know if they were alright. Especially one of them who had wrestled some with suicidal ideation. I don't even know if she's still alive, to be honest, so that's why I wanted to hear from her again.
But as time goes on, I don't feel any spiritual fruit in this matter. I'm not sure I sense the presence of the Holy Spirit in guiding me to focus on this situation. I've even asked Him directly if I should or not, though I have had a hard time picking up on what His voice is telling me, whatever His answer may be.
So I find myself wondering often, even more strongly than before, if I should just make my peace that I will never hear from these friends again, and let them go, clean them from my mind. I can still include them in my general prayers about salvation for my unsaved friends/acquaintances, but I question if I should still focus on these three individuals specifically and getting an answer on them. It just seems frustrating to invest hope when I am not sure it is God's will in the first place.