RegionPlot3D[
  2 x1 + x2 + x3 + 2 x4 >= 12000 &&
  3 x1 + 4 x2 + x3 + x4 >= 35000 &&
  50 x1 + 20 x2 + 12 x3 + 30 x4 >= 205000 &&
  x1 >= 0 && x2 >= 0 && x3 >= 0,
  {x1, 0, 25}, {x2, 0, 15}, {x3, 0, 3},
  Mesh -> All, PlotPoints -> 25, AxesLabel -> Automatic]