Each Placement3D has 2 read-write properties which describes its transformation : 

Matrix3D Placement3D::AbsoluteTransformation - absolute transformation 

Matrix3D Placement3D::RelativeTransformation - transformation relative to a parent object 

The properties are represented by 4x4 transformation matrix : 

M11      M12      M13     M14 

M21      M22      M23     M24 

M31      M32      M33     M34 

OffsetX OffsetY OffsetZ M44 

Here is an example of setting transformation matrix to a 3D object:

Vector3D oVector3D = new Vector3D();
oVector3D.X = 3.0;
oVector3D.Y = 4.0;
oVector3D.Z = 5.0;
Quaternion oQuaternion = new Quaternion(oVector3D, 2.0);
Matrix3D oMatrix3D = new Matrix3D();
oMatrix3D.Rotate(oQuaternion);
oMatrix3D.Translate(new Vector3D(1.0, 2.0, 3.0));
oComponent1.AbsoluteTransformation = oMatrix3D;

It is also possible to move 3D object using Move() method :

oComponent1.Move(1.0, 2.0, 3.0);

For more details about Matrix3D structure, please refer to MSDN webpage. 

It is also possilbe to transform objects by means of mates - see chapter Mates for the details.