If you unroll the fence, you find it has a length equal to the circumference of a circle of radius 10 m and an average height of 3 m. Its two-sided area will be
... A = 2·(3 m)·(2π·(10 m)) = 120π m²
Then the amount of paint required is
... (120π m²) × (1L)/(100 m²) = 1.2π L ≈ 3.77 L