Hi Julius,
I would slightly disagree here. Usually you will have firewalls between different protection zones, this could be only the zones you listed, however it is not uncommon and sometimes even required that you also have firewalls to split networks for prod and dev/cons systems to better control the traffic and the access to the prod systems. Especially for systems holding credit card data, this may even be required by standards. Often you will also find similiar setups for sensitive systems like HR systems. I completely agree however, that for an SAP ABAP systems having a firewall between the R3 system and the DB systems is not very helpful from a security point of view.
BTW: there are some recommendations from SAP to this regards in the security guides. Especially with regards to the placement of firewalls there is also some info here.
Kind regards,
Patrick