n=2;Y=1;d=0.001;f[x1_,x2_,x3_]:={(x1-x2)^n+(x2-x3)^n+(x1-x3)^n}^(1/n)-Y*2^(1/n);ContourPlot3D[f[x1,x2,x3]0,{x1,-1-d,1+d},{x2,-1-d,1+d},{x3,-1-d,1+d},RegionFunctionFunction[{x1,x2,x3},x1>=x2>=x3&&0.95<=x1+x2+x3<=1.05],AxesTrue,AxesLabelAutomatic,PlotLegendsPlaced[{"n = 2"},Right],ContourStyle->{Green},BoundaryStyleDirective[Green,Thick]]
Out[]=
