If you are substituting into the usual light set say .012, .016, ... I would recommend going a lot lighter than the gauge of the wound 3rd string (probably a .024?). I use a .018 for a couple of reasons. Primary is that I tend to want to really bend the string a lot and the light gauge definitely makes that easier. Another reason is that the fat plain strings seem to be harder to keep in tune, and, also in that area, an .018 is close to the gauge of the core of a wound .024 and, because it is the stretching of the core that governs the intonation, the plain string will intonate pretty close to the same as the original wound string. So no need to have the saddle adjusted if that's an issue to your ear.
But don't take my word for it. Buy a few strings of different gauges, maybe .018, .020 and ,022. Try them all and see which you like. This will teach you a lot about how to hear and feel small differences in your guitar.
: I realize there are many opinions regarding
: using an unwound 3rd string. I have not
: experimented with this yet, but I am
: seriously thinking about trying it out next
: time I change my strings. My question is :
: Who makes such a set (preferably Phosphor