[Lambda][n_] := ((n [Pi])/L)^2

[ScriptCapitalX][
x_] := [ScriptCapitalC][1] Cos[Sqrt[[Lambda][n]] x]

[ScriptCapitalT][t_] := 50 E^(-[Lambda] t)

[ScriptCapitalU][x_,
t_, [ScriptCapitalN]_] := [ScriptCapitalX][x] [ScriptCapitalT][t]

Plot[Evaluate@
ReplaceAll[
Table[[ScriptCapitalX][x] [ScriptCapitalT][t], {n, 1, 3}, {L, 1,
 1}], {[ScriptCapitalC][1] -> 1, [Lambda] -> 1, t -> 0}], {x, 0,
1}, PlotLegends -> "Expressions", AxesLabel -> {"x", "t"},
PlotLabel -> "2D Plot Behaviour for [ScriptCapitalU](x,t)"]

Plot3D[ReplaceAll[
Table[[ScriptCapitalU][x, t, [ScriptCapitalN]], {n, 1, 3}, {L, 1,
1}], {[ScriptCapitalC][1] -> 1, [Lambda] -> 1}], {t, 0, 1}, {x,
0, 1}, AxesLabel -> Automatic,
PlotLabel -> "3D Plot Behaviour for [ScriptCapitalU](x,t)"]