plotting – I need help please

You have a number of problems in your code.

Mathematica reserves uppercase N and O, you can’t reassign them (well, at least not easily). You don’t really need them.

Your code doesn’t define 3 plots, so the Show can’t show 3 curves.
That said you can avoid the Show completely.

Also, you’ve used Insert instead of Inset in the Epilog assignments.

c = 2.998 10^8;
h = 6.62607015 10^-34;
Kb = 1.380649 10^-23;

i((Lambda)_,
   T_) := ((2 Pi h c^2)/((Lambda)^5 (Exp((h c)/((Lambda) Kb T)) -
      1)));

Plot({i((Lambda), 3000), i((Lambda), 4000), 
   i((Lambda), 5000)}, {(Lambda), 0, 2.5*10^-6},
  PlotRange -> All,
  PlotStyle -> {
    {Thickness(0.005), Green},
    {Thickness(0.005), Blue, Dashed},
    {Thickness(0.005), Orange}},
  Frame -> True,
  FrameLabel -> {"(Lambda)(m)", "(watts/m^3)"},
  Filling -> {2 -> Axis, 3 -> 2},
  FillingStyle -> {1 -> None, 2 -> Automatic, 3 -> Orange},
  GridLines -> Automatic,
  
  Epilog -> {
    Inset(
     Framed(Style("T=3000K", 10), Background -> White), {8.23*10^-7, 
      0.75*10^13}), 
    Inset(Framed(Style("T=4000K", 10), 
      Background -> White), {8.23*10^-7, 1.64*10^13}), 
    Inset(Framed(Style("T=5000K", 10), 
      Background -> White), {8.75*10^-7, 3.90*10^13})}
  ) // Quiet

enter image description here

You need to do some additional formatting of the Epilog style and the placement, but I’ll leave that to you.