A tex/Builder set to _3D makes a 64 by 64 by 64 texture from code nodes: from code/Globals' UVW, the distance to three centres, softened at each sphere's edge, the largest of the three kept. Open the texture panel to see it slice by slice or raymarched. geo/VolumeFromTexture brings it into the viewer as a volume, rendered as fog on one side and meshed on the other.
tex/Builder code/Globals tex/Output geo/VolumeFromTexture