How do i tell smsd to try a specific device first? I tried this wthout luck:
echo -e "To: +12345678\nModem: GSM3\n\n$(date)" > /var/spool/sms/outgoing/test-sms
Where GSM3 is the prefered device.

The modem header is an informative marking of which modem has processed or received the message.

The selection of the transmitting modem can be made via queues and providers. For example, in a two-modem system, the smsd.conf is like:



The modem reads the given queues in the given order. This can affect the processing speed in systems with a lot of messages.

The message file is placed in the outgoing folder as usual and equipped with the header Queue: <name>, which causes the message to move to the desired queue.

Specifically, you asked "...try a specific device first?". Queue selection is final, so other modems no longer process the message. If the selected modem fails in transmission, the message is moved to the failed folder, from where the own external process must move it to another queue, for example available to all modems, if so desired.