RegionPlot3D[
((x^2) + (y^2)) <= 9 &&
(z >= 0) &&
z <= Sqrt[x^2 + y^2]/3,
{x, -3, 3}, {y, -3, 3}, {z, 0, 4},
AxesLabel -> {x, y, z},
Mesh -> True,
WorkingPrecision -> 120,
MaxRecursion -> 5,
PlotRange -> {{-4, 4}, {-4, 4}, {0, 4}},
PlotPoints -> 400]