Inspired by the documentation for AxisObject:

ternFr[xmax_] := Graphics[{AxisObject[Line[xmax {{1, 0}, {0, 0}}], 
           {0, 100}, AxisLabel -> Placed["Component A", 
               {0.5, {0.5, 2.5}}], RotateLabel -> Automatic, 
           TickDirection -> "Inward"], 
         AxisObject[Line[xmax {{0, 0}, {0.5, Sqrt[3]/2.}}], 
           {0, 100}, AxisLabel -> Placed["Component B", 
               {0.5, {0.5, -2.}}], RotateLabel -> 60*Degree, 
           TickDirection -> "Inward"], 
         AxisObject[Line[xmax {{0.5, Sqrt[3]/2.}, {1, 0}}], 
           {0, 100}, AxisLabel -> Placed["Component C", 
               {0.5, {0.5, -2.}}], RotateLabel -> -60*Degree, 
           TickDirection -> "Inward"]}, 
       PlotRange -> xmax {{0, 1}, {0, 1}}, Frame -> True];
Show[DensityPlot[x^2 + x*y + (100 - x - y), 
     Element[{x, y}, Triangle[100*{{0, 0}, {1, 0}, 
             {1/2, Sqrt[3]/2}}]], Frame -> None],
  ternFr[100]]