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In[]:=
RegionPlot3D[{Abs[3x-y-z-2]<=0.1,Abs[x+2y-3z-12]<=0.1,Abs[2x+y+3z-9]<=0.1},{x,-3,3},{y,-3,3},{z,-3,3},PlotPoints->100]
Out[]=
In[]:=
ContourPlot3D[{3x-y-z==2,x+2y-3z==-12,2x+y+3z==9},{x,-3,3},{y,-3,3},{z,-3,3}]
Out[]=