Body heat is lost by all three methods. Radiant energy loss is very small, for practical purposes insignificant. Clothing with reflective threads or foil linings are intended to trap heat by reflection of radiant energy. Unless these fabrics are used with good insulating materials they are of little value. In general any product relying on this principle to conserve body heat should be avoided.
All materials conduct heat.
The theory of insulation is to use fabrics that are very poor conductors, and therefore good insulators. Virtually all outdoor clothing uses the same insulator, air. It is widely available low cost and a poor conductor of heat. Unfortunately air needs a little help. Conduction and convection will still draw away heat.