As an alternative, you can also use Image3D
to visualize 3-variable functions:
valueInterval = Through[{MinValue, MaxValue}[
{x^2 + y^3 + z^4, And @@ Thread[-1 <= {x, y, z} <= 1]}, {x, y, z}]];
Outer[
Function[{z, y, x}, If[x > 0 && y > 0 && z > 0, 0,
4 Mod[Rescale[x^2 + y^3 + z^4, valueInterval], 1/5]
]], Reverse@#, Reverse@#, #] &@
Range[-1, 1, .02] //
Image3D[#, ColorFunction -> "RainbowOpacity",
Boxed -> True, Axes -> True, AxesOrigin -> {1, 1, 1}/.02,
AxesLabel -> (ToBoxes[Style[#, 20]] & /@ {x, y, z})] &

Or a 3D version DensityPlot
:
Outer[
Function[{z, y, x}, If[x > 0 && y > 0 && z > 0, 0,
Rescale[x^2 + y^3 + z^4, valueInterval]^2.2
]], Reverse@#, Reverse@#, #] &@
Range[-1, 1, .02] //
Image3D[#, ColorFunction -> "RainbowOpacity",
Boxed -> True, Axes -> True,
AxesLabel -> (ToBoxes[Style[#, 20]] & /@ {x, y, z})] &

ContourPlot3D
. $\endgroup$ – Szabolcs May 15 '13 at 23:37ContourPlot3D[x + y + z, {x, -2, 2}, {y, -2, 2}, {z, -2, 2}, Mesh -> False, ColorFunction -> "Rainbow"]
. It sounds like you're not sure what you want to draw on the screen. Try to think up a way to visualize the function, then ask about how to implement that. $\endgroup$ – Szabolcs May 15 '13 at 23:47Manipulate
. $\endgroup$ – Szabolcs May 15 '13 at 23:48