A single discount equivalent to three successive discount of 6%, 15% and 14% is:

$$\eqalign{ & {\text{Discount }}\% = x + y - \frac{{xy}}{{100}} \cr & {\text{2 years}} \to {\text{discount }}\% \cr & = 6 + 15 - \frac{{6 \times 15}}{{100}} \cr & = 21 - 0.9 \cr & = 20.1\% \cr & {\text{3 years}} \to {\text{discount }}\% \cr & = 20.1 + 14 - \frac{{20.1 \times 14}}{{100}} \cr & = 34.1 - 2.814 \cr & = 31.286\% \cr} $$