Try the link and see if that resolves your problem.
The way I finally stumbled onto that was giving WolframAlpha
triple integral
and then editing their example to use your function.
I wish I could have given you an exact reason it didn't work for you and point to the place in the reference manual for Wolfram Alpha that showed you exactly why your query didn't work and exactly what syntax to use to make it work. If you ever find that manual then please let everyone know where it is.
