Although there are three mechanisms of heat transfer, a medium may involve only two of them simultaneously.
Heat transfer is only by conduction in opaque solids, but by conduction and radiation in semi transparent solids.
A solid may involve conduction and radiation but not convection. A solid may involve convection and/or radiation on its surfaces exposed to a fluid or other surfaces.
Heat transfer is by conduction and possibly by radiation in a still fluid (no bulk fluid motion) and by convection and radiation in a flowing fluid.
In the absence of radiation, heat transfer through a fluid is either by conduction or convection, depending on the presence of any bulk fluid motion.
Most gases between two solid surfaces do not interfere with radiation.
Liquids are usually strong absorbers of radiation.
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