We want to communicate with 3 Siemens PLC's using the MPI Protocol. The MPI adapter XBTZ929 is connected on port COM1.

3 PLCs to 1 target cannot be used with the MPI Adapter. The MPI Adaptor is an RS232C Serial Interface, which means it is a Point-to-Point protocol.
This is not a defect. It is the specification of RS232C serial protocol. However, the IO Manager should not allow additional EQUIPMENT to be added when using the MPI Adapter Driver. There should only be one EQUIPMENT under the driver node.

If the customer needs to use 3 PLCs, I would recommend them to use the Siemens MPI/PPI Protocol instead. This protocol is RS422 2-Wire which supports 3 PLC to 1 target connection. Switching over to this protocol is just a matter of reconfiguring the project and creating a new cable.