Actually, the technique above is a 2d wavetable. You're driving around this 2d surface - that is, you can control your longitude and latitude - and the height of the land is the output.
A standard wavetable is like a normal graph - you can control where you are on the x-axis, and the y-axis is the output.
A 3d wavetable would be more like... umm... a gas tank full of (stationary) smoke in a funny marbled pattern, where you move around the tank (you can control longitude, latitude and height) and the "result" is the opacity of the smoke at that point.
I'm not sure if a synth with a 3d wavetable exists. How feasible it is depends on how the wavetables morph into one another. If it's simply crossfading, then a 3d wavetable would not be complicated to implement at all and is basically tantamount to using a 3 oscillator synth and controlling the level of each. That wouldn't really be very exciting and you can rig up most synths with some kind of X-Y MIDI control to fade between oscillators. However, take the case of Massive. The wavetable position knob doesn't just crossfade between two wave shapes... in the more complex wavetables it's actually crossfading through many more (I think there are up to a hundred nodes or something like that, but I'd have to check). This is necessary in order to create interesting "morph" effects, otherwise it just sounds like, well, crossfading.
Now, if you have two axes instead of one, you could end up with that number, squared - so a hundred wavetables on one axis, a hundred on the other - that's 10,000 wavetables for one oscillator. Each of those being a few hundred samples, or more.
And even a "single" wavetable - no morphing - isn't as simple as it seems. Digital synth developers always need to avoid aliasing somehow, and with wavetable synths that often means using a different table (containing fewer or more harmonics) depending on which note on the keyboard is played. This means that even a single wavetable synth could actually be using several wavetables "under the hood".
To cut a long story short, to do it properly would take up rather a lot of memory and CPU. Of course, if you just want to fade between 3 wave shapes, you can just use a synth with 3 oscillators and adjust the level of each.