i'm on pc.
For my game smiling Larry and the Scribe were encountered at random points as road walkers.
I got The Scribe before the patch by using the console because after meeting him he didn't turn up.
I followed him for a few paces and he would just disappear.
I got Smiling Larry successfully recruited post patch the way the game originally intended.
He walked all the way and I was able to follow him which was fun because he went the long way round passing a number of enemy bases and engaging in fighting.
In my game Doc Anderson and Ron Staples appear at a number of campsites around the map where other random encounters occur.
I was able to successfully recruit both of them and both are tending their assigned stores.
However I also see them at the campsites from time to time even though they are supposed to be at settlements.
I discovered that if i don't talk to them at the campsites they remain assigned to their stall at the settlement.
One time when i talked to the Doc at a campsite she disappeared from the settlement, so i loaded a save and avoid talking to them when they appear at campsites.
One thing i note for my games is that meeting Smiling Larry and the Scribe as random encounters is a much rarer occurrence than meeting the other two you mention.