Dear minafogiovanni, please see the ATENA Theory manual for detailed information:
For the geometry, above all Chapter I.
For the 3D volume elements, 3.5 3D Solid Elements. As far as I understand, hourglassing does not come into question as the linear brick elements have 8 Nodes and also 8 Integration Points.
Under typical conditions, locking is excluded by providing enough elements per thickness in bending direction (see also ATENA 3D User's, 188.8.131.52.4 Final Notes on Meshing or ATENA-GiD User's, 5.7.1 Notes on Meshing).
If you have some specific suspition, please follow Troubleshooting, 2.1.1 to send us your model etc.