The plot of 2 variables define the surface in 3D space. To visualize the plot of 3 variables you can use a vector field. In case of your example it looks like 
     
     VectorPlot3D[{x, y, z}, {x, 0, 1}, {y, 0, 1}, {z, 0, 1}]


![enter image description here][1]


  [1]: https://i.sstatic.net/woXm7.png