TheGenerally the plot of 2 variables define the surface in 3D space. ToSimilarly, in the general case the plot of 3 variables define 3D space in 4D space. So, 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}]