$\frac{1.10 + 1.15 + 1.20}{3} - 1 = 14.83\%$

$(1.10 \times 1.15 \times 1.20)^{1/3} - 1 \approx 14.85\%$

$\frac{10 + 15 + 20}{3} = 15\%$

$\sqrt{1.10 \times 1.15 \times 1.20} - 1 \approx 12.3\%$