There are many middleware tools that work as middleware for integrating different applications. There exists a vast variety of them but choosing a correct one is based on the applications being integrated.
For integrating the third-party application with either SAP or Navision, you can use Apache Camel. Also, SAP provides a framework - SAP PI - SAP NetWeaver Process Integration.
Lately, SAP Business One has emerged as the preferred choice for integration of third-party applications with SAP. It is highly preferred by developers as it provides vast options for handling various use cases. Its working is based on either of the two ways: