The Ladybug documentation explains what this input expects a bit more clearly:
Basically, it’s scenario 1 if you don’t have your surface temps (and therefore are assuming surrounding surfaces are temperature of ambient air).
If you do have surface temperatures, you can calculate an average of the different surfaces temperatures weighted by the view factor, from the location at which you are calculating the MRT. This makes more sense then your scenarios 2 and 3, since you’ll notice the inputs for the OutdoorSolarMRT components don’t include the raw surface geometries. Therefore it has no way to understand surface geometry relationships, you have to precompute the surface temperatures beforehand to account for such geometric relationships.