Bob, In most parts of the country the wall assembly should be a minimum of R-19 for a standard frame wall, i.e., drywall, fiberglass batt, exterior clapboard. If you do not have enough insulation it is better to fill the cavity than to add to the outside. R-3 installed on the outside of the clapboard will add little insulation value. The primary benefit of the insulation is that it provides a smooth surface for the application of the siding.
House wrap is designed to break airflow and drafts.
Its performance is questionable, but the cost is so low it is generally worthwhile. The telltale sign of excessive interior moisture is condensation on windows if you are in a cold area. If you do not have enough insulation it is better to fill the cavity than to add to the outside. The primary benefit of the insulation is that it provides a smooth surface for the application of the siding. House wrap is designed to break airflow and drafts.