What is the difference between double glazing and triple glazing?
Triple glazing is usually touted as the best for energy reasons.
Sound reduction in triples can be better than doubles, but it's not dramatic.  Asymmetric glazing (different thicknesses) can give triples an edge by blocking (attenuating) different frequencies, but these are special features not on the average triple.  The larger air gap of a double is a better sound insulator.

Your best comparison, insulation-wise, is to compare the R-value / U-factor of 2 units.  The U factor is nothing more than the reciprocal of the R value.  A window with a R 10 rating will be a U 0.10 (a great window currently.

Triple glazing is the darling of superinsulated homes and has a price to match. They typically only make sense in colder climates where heating costs would justify the expense.  Windows must be paired other areas of energy conservation: air sealing, ceiling insulation.  An energy audit that does a blower door test, paired with a thermo imaging analysis, should be done before upgrading to triple glazing. Some local utilities have discounted energy audits.