[abenedik]

Do you mean SphereVisual3D.CenterPosition as the "centerposition"?

You have two options:
1) instead of applying a TranslatTransform3D on the SphereVisual3D, you can just change the value of the CenterPosition.

2) to get the transformed centered position you can use:

var transformedCenter = modelVisual3D.Transform.Transform(sphereVisual3D.CenterPosition);


Note that transformation are hierarchical - so you can have a hierarchy of objects - for example:

ModelVisual3D
   ModelVisual3D  
      SphereVisual3D
      BoxVisual3D

So if you apply the transformation to the first ModelVisual3D this will transform all child objects, but CenterPosition on SphereVisual3D and BoxVisual3D will not change - the values of those properties are defined in the local coordinate system (and not in world coordinate system).