So I made a picture of the graph and came up with the domain lying in the xy plane. It was bounded by x = 0 and y=3x and y = 3.

y=3 comes from when z = 0 in the xy plane,

So it forms a triangle. It is vertically linear, so y ranges from 0 to 3, x ranges from 0 to y/3. Then finally z ranges from 0 to

Then all you have to do is integrate.