Panel[Manipulate[tick; Dynamic[b = 8]; Dynamic[c = 4];
Dynamic[uMin = -5]; Dynamic[uMax = 5];
KFigE = Dynamic[
Plot3D[dydx /. {B -> b, C -> c}, {x, uMin, uMax}, {y, uMin,
  uMax}]],
Grid[{
{Style["PLOT", "Title"], SpanFromLeft, SpanFromLeft},
{Style["Start:", Blue, Bold, 15]},
{Style["Initial Conditions", Blue, Bold, 15]},
{Style["b", Bold, 12],
 InputField[Dynamic[b], Number, Background -> Lighter[Gray, 0.7],
  ImageSize -> 80],
 Style["uMin", Bold, 12],
 InputField[Dynamic[uMin], Number,
  Background -> Lighter[Gray, 0.7], ImageSize -> 80]},
{Style["c", Bold, 12],
 InputField[Dynamic[c], Number, Background -> Lighter[Gray, 0.7],
  ImageSize -> 80],
 Style["uMax", Bold, 12],
 InputField[Dynamic[uMax], Number,
  Background -> Lighter[Gray, 0.7], ImageSize -> 80]},
{Button["Do it", tick = Not[tick],
  Background -> Lighter[Green, 0.5], ImageSize -> {80, 40}],
 Button["Clear", {dydx = Null, b = Null, c = Null},
  Background -> LightRed, ImageSize -> {80, 40}],
 Button["Export",
  Export[SystemDialogInput["FileSave", "untitled"], KFigE],
  Background -> LightBlue, ImageSize -> {80, 40},
  Method -> "Queued"]}}, Spacings -> {2, 1}],
  {{dydx, 3*(B*x + C*y), "Equation", None},
  Background -> Lighter[Gray, 0.7], ImageSize -> 200},
  {{tick, False}, None}, TrackedSymbols :> {tick},
  ControlPlacement -> Left]]