The solution to the problem is as mentioned by @chris in comments of the question

Replace the `VectorPlot3D[]` command which is

    VectorPlot3D[Grad[rint3[x, y, z], {x, y, z}], 
    {x, -2, 2}, {y, -2, 2}, {z, -2, 2}]

by

    VectorPlot3D[Grad[rint3[x, y, z], {x, y, z}]//Evaluate, 
    {x, -2, 2}, {y, -2, 2}, {z, -2, 2}]

"//Evaluate" command has been added.

The output will be

[![Gravitational Field due to a cube][1]][1]


  [1]: https://i.sstatic.net/sKCVc.jpg