Filling between more than two boundaries

r = ImplicitRegion[
   y <= 11 + x && y <= 27 - x && y <= 1/5 (90 - 2 x) && x >= 0 && 
    y >= 0, {x, y}];
Show[Plot[{11 + x, 27 - x, 1/5 (90 - 2 x)}, {x, 0, 20}, 
  AxesOrigin -> {0, 0}], RegionPlot[r]]

Plot[{11 + x, 27 - x, 1/5 (90 - 2 x), Min[11 + x, 27 - x], 
  Min[11 + x, 27 - x, 1/5 (90 - 2 x)]}, {x, 0, 20}, 
 Filling -> {5 -> {Axis, {White, LightBlue}}, 4 -> {{3}, {None, Yellow}}}]

Depend on @kglr, but only use three functions.

Plot[{11 + x, 27 - x, 1/5 (90 - 2 x)}, {x, 0, 20}, PlotPoints -> 40, 
 Filling -> {3 -> {Axis, {None, Blue}}, 2 -> {{3}, {White, None}}, 
   1 -> {{3}, {White, None}}}]