Built-In Volumes
Use spheres with sectors of varying size removed as the surface slices for a contour plot.
show complete Wolfram Language input
In[2]:=

func = Simplify[
   Sum[Cos[5 Norm[{x, y, z} - {Sin[\[Theta]], Cos[\[Theta]], 
         0}]], {\[Theta], 0., 2 \[Pi] - (2 \[Pi])/3, (2 \[Pi])/
     3}], (x | y | z) \[Element] Reals];In[3]:=

Grid[Partition[
  Table[SliceContourPlot3D[func, 
    s, {x, -2, 2}, {y, -2, 2}, {z, -2, 2}, 
    Contours -> Subdivide[-2.5, 2.5, 2], 
    Evaluate@opts], {s, {"CenterSphere", 
     "CenterCutSphere", {"CenterCutSphere", 
      3 Pi/4}, {"CenterCutSphere", Pi}}}], 2], Spacings -> {2, 2}]Out[3]=


















 
  
  
  
  
  
  
 