One single discount which is equivalent to 20%, 10% and 5% is given by:

31.60%

Let Marked Price = 100x

According to question ,

Price after 3 successive discount = (100-5)% of (100-10)% of  (100-20)% of 100x

= 95% of 90% of 80% of 100x

= \(\frac{95}{100}\) × \(\frac{90}{100}\) × \(\frac{80}{100}\) × 100x

= 68.40x

Required discount = \(\frac{100x - 68.40x}{100x}\) × 100

= 31.60%