Pars = {ep -> 0.5, f1p -> 0.3, f2p -> 0.1, dp -> 0.05, q -> 0.1,
   en -> 0.4, d -> 0.1, Q -> 0.1, f2n -> 0.3, f1n -> 0.4, a -> 0.05,
   N1 -> 0.5, N2 -> 0.5}; #Parameters

PlotREq1 =
  Plot[R = S /. Pars, {S, 0, 0.9375},
   PlotRange -> {{0, 3.5}, {0, 2}}, PlotStyle -> {Red, Thick, Dashed},
    GridLines -> {{0.9375}, {}}];
PlotREq2 =
  Plot[R = (a + d)/(en f1n) /. Pars, {S, 0.9375, 1.4375},
   PlotRange -> {{0, 3.5}, {0, 2}}, PlotStyle -> {Red, Thick, Dashed},
    GridLines -> {{1.4375}, {}}];
PlotREq3 =
  Plot[R = ((a + d) (f1p - f2p))/(en (f1p f2n - f1n f2p)) /. Pars, {S,
     1.4375, 2.3}, PlotRange -> {{0, 3.5}, {0, 2}},
   PlotStyle -> {Red, Thick, Dashed}, GridLines -> {{2.3}, {}}];
Show[PlotREq1, PlotREq2, PlotREq3]

Show[PlotREq1, PlotREq2, PlotREq3, GridLines -> {{0.9375, 1.4375, 2.3}, {}}]